The first/leftmost field in a table is always the primary field, but you can change the values in this field if you want to change what shows up when you link to it from another table.
A quick way is just to copy a whole column of data from a different field in your table and paste it into the primary field.
I prefer to turn my primary field into a formula field that refers to other fields on the table. That way if I want to change the information that’s displayed, I can just go in and change the formula. It can be a simple formula that just shows the same data as a different field, or it can become more complex and combine data from multiple fields.
Does this answer your question or is there something more specific you’re trying to do?
