HL Segment - Hierarchical and Parent ID Number

When mapping X12 856 transactions, how are others populating the HL segment Hierarchical and Parent ID numbers?

For the Hierarchical ID, I created a UDF counter that increments for each HL element. I have considered the same for the Parent ID. However, is there a built-in attribute to assign the Hierarchical ID numbers? For example, there is the "$B2B_SEG_COUNTER" that counts all segments. Is there also one to perform the HL count?

BTW, where is the $B2B_SEG_COUNTER documented? I looked everywhere and could only find this value in the "B2B MAPPING KIT" EDIFACT mapping example.

Thanks!