Meaning of SLA Source : Account Segment for application : Payables

Hi Team, 

We have a SLA requirement with the below rule : 

Accounting Class : Item Expense

Journal Line Type : Item Expense

If Account segment of the combination for the item expense = 21710 

=> Site segment should be 0000

So we have created a custom ADR for Site segment with the below condition : 

Priority  : 1 

Value Type : Constant 

Value   : 0000

Condition : 

================

Sequence  : 10

Source   : Account Segment 

Value Type : Constant 

Value   : 21710

But as checked in all cases its not working. 

Account segment is derived from a custom source. 

So through the SR we want to ask the below questions : 

a) What is the meaning of SLA source : Account segment for application : Payables. If this source is taken then how system derives the segment value.