Settlement rules are nothing but settling the account for a 
cost center.Which WBS are account assigned the cost of 
corresponding WBS should be settled to the respective cost 
center assigned for that WBS.Settlement rule is assigned in 
settlement profile in which you will define (like assect)
the rules of settlement.

In SAP most cost objects - other than cost centers - must be
"settled" at period end. By means of the settlement the
costs incurred for the cost object (project, internal order,
...) are debited to other cost objects and/or G/L accounts.
The settlement rule, a table maintained in the cost object
master data, describes to SAP how the costs will be settled:
receivers, percentages, full/partial, etc.. 
Cost are settled when the user starts the settlement
procedure at period end.