Tracking specification lines - when do they get created?

davmac1

In a sales order, when you display item tracking, you can actually get the reservation entries displayed in a temp table version of tracking specification. What triggers the sales order to generate tracking specification records? Do you have to use directed Put-away and Pick?
I am planning to use the quantity handled (base) in a custom external app, and the reservation record does not have that field.
I would prefer to automate current functionality rather than add a custom field to reservations and control it that way.