Customer Part Cross Reference is for when the Customer has their own part number and for whatever reason you have a DIFFERENT part number for the exact same item. I use this one a lot.

I’ve really only used Internal Cross Reference at implementation time when there have been multiple part numbers for the same item in a previous system… picking one to be the “real” one going forward and linking the others. I had an implementation last year where some parts had literally half a dozen different part numbers scattered around the company (needless to say their inventory was a nightmare).

Manufacturer Part Cross Reference is when (for instance) you get a requirement to build something and the customer wants one of the components to be “equivalent to XXX (manufacturer name) Part Number YYYYY”… and your supplier has a DIFFERENT part number for it… and you may (or may not) have yet a third part number for it. I haven’t used that one for quite some time.