debug on atp check functions

Dear

There are two functions which are 'ATP_CHECK_BASIS' in program 'SAPLATP3 ', and the other 'AVAILABILITY_CHECK_CONTROLLER' in program 'SAPLATPC'; If i set break-point for either of them, the program will stop when i create sales order or delivery note through VA01 or VL01N. I want to know the logic relevance behind those two functions or two programs. Why does sap check both of them when process creating orders? anyone could give me some explaination?

I did do some debugs, however still couldnt find single clue. thanks！

T.P
best regards