The most recent CD tag file is used by HFSLIP to determine the Service Pack level. It is important to know the Service Pack level in order to integrate the correct version of files in hotfixes (among other things).

The CD tag file resides in the root of your installation CD and should be copied over into the SOURCE folder. If HFSLIP does not detect a CD tag file, it assumes that the Service Pack level is "0" or "Gold".

Windows XP Professional SP1: win51ip.SP1
Windows XP Professional SP2: win51ip.SP2
Windows XP Home SP1: win51ic.SP1
Windows XP Home SP2: win51ic.SP2
Windows Server 2003 Standard Server SP1: win51is.SP1
Windows Server 2003 Advanced Server SP1: win51ia.SP1
Windows Server 2003 Datacenter Server SP1: win51id.SP1
Windows Server 2003 Web Server SP1: win51ib.SP1

This does not apply to Windows 2000. For that OS, it is assumed that the source is patched with SP4.

And if I have
-WIN51
-WIN51ip
Without any extension ?

That either means your source wasn't patched with a Service Pack or that the Service Pack wasn't properly applied (or you deleted the SP tag file by accident).