The HP service tags explained, simply.          

This is the first question asked by HP or any technician worth salt.  (if he can't see your PC or touch it , he/she is blind) so asks you for the TAG data first.
If not?, all answers will go sideways fast.
The same is true for Dell PCs (a good US brand to) I buy USA !

All HP PCs have a service tag, but maybe not that 29 year old COMPAQ that HP did not own then, not until 2001 year,  so I'd bet accurate TAG data started then, at HP.

The tags are located on  any side of  Desktop (PC) (not front) but on top or sides.
The Laptops are located on the bottom or inside the battery well , yes, battery popped out and removed, look inside.
Later HP got the fun (not) idea to put the tag only in the BIOS INFO screen, so if BIOS is dead, you can't find it,  is this fun, NO !

The tag is super important to any skilled technician , as it has codes there telling him or her what options were installed new.
Just base line new PC had vast numbers of options, and coded in the p/n: field seen below.
  • Different motherboards inside (MOBO)  with different chips it, for example?
  • Different CPU,'s Intel or AMD. (main Central Processing Unit)
  •  GPU (grapic processor chips)  Mine has 4 choices, new,  others have more.
  • WIFI  (2 to 4 choices) (wireless wifi or even wireless 3G or 4G cards installed)
  • Ethernet (about 2 choices) Wired internet connections (robust)
  • 2 or more LCD screens choices, even 3D options.
  • 50 keyboards or more options, 1 for each country sold.
  • Card readers, fingerprint readers, for sure lots of  other options new, bluetooth, DVD or BlueRay options, and more.)

Show and tell time. The HP Generic tag..  line 3 and 5 are the most important, 

Real Example #1  As you know know, DV5 means little at all, what matters is all numbers here,  (p/n and that dash number see at the red arrow)
A real laptop, DV5,  See that #ABA line, this is super important information as is the 1250us (us = usa , see more suffix codes here that I collected.)

Next Example #2 is MINE (we have 4 of these in the family and spares we buy them in bundled lots all dead, and we repair then upgrade them all to SSD drives.
This is my favorite Elitebook,  (it has 4 GPU chip CARDS (yes cards) options inside, yah , in a Laptop !) This has a LED screen that does NOT go dead like CCLF did from 2000 to 2010.
My Laptop 8540w (w means workstation grade with high end faster i7 processor)

Example 3: one more much newer Laptop bought at HP store (refurbs) Most Refub HP direct buys are actually new, but returns from confused customers)
Next is Wifes HP laptop, note the battery must be removed. (the battery above is tilted out. and in the well there is the official service tag. (the #ABA number tells me the options loaded)

Example 3: I like real photos for examples.
The Desktop and Workstation below has this tag. Own a real desktop is best for 2 reasons, parts inside are standards for fit and size, ATX spec. And are vastly more easy to diagnose and repair.  (2 screws to get inside, is nice)

Note the word Pavilion, is on both desktops and laptops so that word really means nothing much; but that tag, sure, lots of facts there. !\
My HP XW4600 work  station,  the tag is on top front silver colored and just  in front of the Microsoft COA tag  W7 pro.
To know what really is inside any Destop, one most take off that left side panel, and LOOK, and see what add-in cards are put there, in those PCI-e slots. What is there matters big time.!
Cards have names, (makers name) and a model, this to is important. (write them down , keep it handy for next time)
Even one model PC can have many different motherboards inside, even HP boards, or Intel made is possible but the Service TAG , tells that story clearly if you look it up. (or look it it with eyes and read the mobo tag inside)

Later the marketing (?) guys at HP  got rid of those ugly useful tag's, and all you get is this below. (after 2010 I guess)
If the PC is dead, how can we read the dead page below, gee ask HP , would love to hear there answers.
Good luck reading  the BIOS page on a DEAD PC.  (thanks marketing pukes for that blunder?)

NEXT UP , is the new HP way of hiding the TAG. Line 2 and 3 below are key facts to finding DRIVERS or parts that fit this PC,  do not buy parts until you know what fits and what does not.
A typical laptop may have 100 mother boards or more, using intel parts or AMD or any of 5 wifi cards, or Sound chips. (and variants by COUNTRY huge)

This is the BIOS main Informatation page (power on from dead cold boot, then hammer gently (tap,tap,tap) the ESC key and see a  1st level BIOS menu, and the pick F1 and behold this page)

Now a blast from the past, 1989 (or near)  LTE 286/286/486 , some with a WHOOPING 640kB of RAM, remember BILL GATES, said this is all you will ever need, ahahahahahahahahaha
Compaq LTE.
 I can not decode any of these , nor can HP, now. The LTE line even ran to 1995.  (most are in the land fill now, RIP OLD COMPAQ's)
The 4/40CX can be seen here, She is booting DOS ! (at Wiki)

A very old horizontal Desktop, remember the first IBM PC's  ? XP ran from 2001 to 2007 (Jan)

LTE 386, wow. ( The dark ages of Laptops here) And pure JUNK.

MONITORS old and problematic to the extreme.
Sold in many sizes.
 This is L1702 monitor tag.  Sub model PE1232
Here is an  old and now dead, LCD monitor tag,  this Monitor is dead due to dead CCFL back lamps.  (most of these failed long long ago, even 8 years ago)
Made in the bad years, 2000 to 2010
This model has the silly internal power saver feature and  a sleep timer , turn both off , as seen here , under monitors.

Nest up is, one more old HP/Compaq, CQ-410us (  note the 410us,  there are a huge number of CQxx laptops and the sub codes 410 
Even old CQ20 on up.
See that #ABA code, that tells the HP helper  person what options it had DAY1.  (VAST Variances exist, sound, network and GPU and CPU chips options exists)
The short name CQ61 means little in fact.
There are CQ61 (CQ20,30,40,100,200,300 and 400 sub series) sold.
This CQ61 is 2008 model, 10 years old today, and some had VISTA installed.    (2000 to 2010 CQ, you can get system INFO like this  "While holding down the Ctrl and Alt keys, press the S key on the keyboard." Ctrl+Alt+S"

Here below is the ABA codes on series 400 above. seen on the HP Parts Surfer pages.
( On some models there are 3 pages full of sub model codes, (huge) and below is just USA, if the PC is from other countries, the list goes to well insane huge.)
As you know know PCs are complex and even more complex are the  OPTIONS sold day1, and world wide too.... That is why folks can not help you lacking these facts conveyed to them..

NOW,  A TIME WARP (A blast from the Past)?
Back to 1989-2000 year (almost 2 decades.   (the Compaq 1010 is about the oldest 20 decade old Compaq) Below is close.
Most of these long ago hit the land full as trash or sent to the 3rd world countries, (the CCFL backlamps inside are again DEAD, dim, yellow or intermittent as H3LL)
The batteries will be dead, as will be the RTC coin cell to.
The Compaq, 1200 laptop.  (12xL2)

We call  this, Windows 98 Suck edition then, (due to USB is dead, more)
This is not a service tag it is a Microsoft COA tag. If buying any used PC, make sure it has W7/8/8.1/10 here, or its mostly a junk PC, ready for recycling.

end show and tell.
 (of TAG data examples)
 Do not ask for help lacking the full model number or you will get the wrong answer almost every time,  how fun can that be?  Even share photos, or your TAG,  OMG! Oh my golly gee, YAH THINK ?
All tags this page are neutered (redacted) for privacy.
Some times we all feel like this.

version 1.  5-1-2018