Checking rule determination in MRP

Dear friends.

when I run T-CODE: MD04, to check the STOCK/REQUIREMENTS LIST for my certain Material, in the menu "environment-->ATP Quantities-->the scope of check", I find the checking rule is 01

what I am confusing is the checking rule, from where can it be determined as "01" here?

can I change the checking rule for it? and where can I make customizing?

Thanks .

Tang.