Earlier availabilty of material

Dear ALL

when I am having multiple source of supply for single material .while planning it select one source of supply . what are the constrained it checks other than procurement priority .

example : Having material MAT1 with source of supply X,Y,Z while planning it selects X source of supply because material is available earlier other source of supply Y,Z .what are constrained it checks for earlier availabilty ?

Regards

Jaisree