Perquisites for enhancing a datasource with a field

Dear Extraction gurus:

This should be a basic question.

When we are enhancing a BI Content datasource with a field from another table, then that table must be somehow related to the primary table of the datasource. Isn't this correct?

For example, we can enhance 2LIS_11_VAHDR (VBAK is the primary table for this datasource) with a field from VBAP table becuase VBAP is related to VBAK through VBELN.

But we cannot enhance 2LIS_11_VAHDR with a field from JSTO table because there is no relation between VBAK and JSTO. In other words, the tables have no common unique field.

Please verify that this is correct.

I hope that made sense.