Which AI should I use to encode your internal part number or SKU in the barcode for internal tracking?

The GS1 Application Identifier (240) can be used to enable identification data other than the GTIN to be represented in the GS1 data carrier. It is a cross reference to previously used catalog numbers. See section 3.5.3 of the currentGS1 General Specifications document for more information. 

Of note, for any AIs applied with a UDI the US FDA has requested that all UDI relevant AIs are placed first before any company or industry specific AIs.