I believe you are referring to the STOCOU field on the STOCK table. I don't know the full extend of it. But I believe it's a unique identifier for each stock line. If you check the STOCK table index, the only index does not allow duplicate STO0 includes STOCOU in it. That implies there are situations where you may have more than one stock line for stock that's identical in every way. Base on X3's data model diagram, the one table that links with STOCK table using STOCOU is the STOALL table (detail allocation table), which makes sense because you could have multiple allocations for the same stock line, in which case would require breaking one stock line into multiple ones with STOCOU field.

I hope this makes sense.