9020 vPro/AMT enabling asks for service tag ?

Hello, our office just bought a batch of brand new optiplex 9020 with the vPro feature, so while configuring them for the first time we land on this prompt (attached picture).

Choices seem fair-enough explanatory, however it asks for a service tag... what is it ? Something to look for on a sticker ? Or something our IT department just need to assign and store ? Is there any standard to follow in order for the prompt to not complain?