Membership parent_no--what is it good for?

So I have a bit of a n00b question...

Looking at TX_CUST_MEMBERSHIP I (naively) assumed that all renewed/reinstated memberships would list the previous membership for that customer (cust_memb_no) as the parent_no for the current membership.

Or as the Tessitura Table Structure states:

0 0 1 13 80 AKM 1 1 92 14.0 96 800x600 Normal 0 false false false EN-CA JA X-NONE

1.1.1.              parent_no int NULL (points to a row in this table that is the current row is a renewal for)

Based on our data, this isn't consistently the case. So I'm puzzled as to what I would use parent_no for in practice. (Although in theory it seems very useful.) Seems like any custom report on renewals would need to rely on customer_no, and not parent_no as I had hoped.

If it's not that, what is it supposed to be?

Thanks in advance.

Michael