hello
from v2 , it was a option in invoice edit mode showing if the line are attached to a service or not it was just a asterisk in the line end (*) .
this asterisk was very helpful for staff to know if the invoice has any relation with a service , now we can't find wich invoice we should change over due date to prevent service suspension .
a realm scenario , we have a service X , the system generate a invoice in their renewal date set , the client sometime order the same service for this service, the staff create a invoice with the same label and price , and we give the client some time between the two invoice, is some case the client request changing the due date , the staf can't determinate wich invoice to edit , they need to change them all now,(5/6 invoices) , and normally they can change just the one attached to the service .
sorry, v2 was wonderfull for our work just for this little tips in core