GTIN+AI(s)=UDI, what AI(s) must be in the PI of the UDI?

It is up to the labeller of the device to determine which PIs are parts of a particular UDI based upon their production control process, and possible UDI regulatory requirements that should be checked accordingly.

The labeller’s determination of which PI to use will determine which AIs to use. For instance, AI (10) for Lot/Batch, AI (11) for manufacture date, AI (17) for expiration date, AI (21) for serial number. Please refer to the section 3.2 of the GS1 General Specifications for an exhaustive list of GS1 Application Identifiers.