Service Tag is essentially your computer's 'serial number', set/locked in the BIOS by Dell, and is the primary number that Dell uses to track your system for support and billing purposes.
Asset Tag is a secondary tag, set by the user in BIOS, that can be used by organizations to track a system for inventory purposes.
The Service Tag can ONLY be set using a special non-public utility from Dell. Typically if your motherboard is replaced, you can boot to the BIOS and have a single attempt to set your Service Tag to the Service Tag of the machine/chassis. If not, then the utility is needed.
having replaced my xps 15 9550 mother board with a supposedly new board im having boot up inconsistencies, i have noticed that my service tag has changed without my input and the asset tag is as previous , bios upgrades do not update, laptop does not restart, cold start up works half the time. should both tags be the same, if so which one should be changed and how and or should i do a clean install of windows, i have down loaded the dell command configure wizard to edit the asset tag if needed. or do i have a completely different problem. laptop works fine once booted up. thanks in advance