Hi Ramesh,
I am not sure about the Rule control key...But, Rule control is what defines how your Product Substotution should behave. For example if you ordered product 1 and substitute product is Product 2 then how system should start doing ATP check will be decided by Rule control
If you define your rule control as as Top down approach -> system will start looking for stock of product 1 and then if product 1 cannot fulfill the requested quantity then Product 2 will be substituted.
If you define your rule control as Bottom up approach -> system will start looking for stock of product 2 first and if shortage is found on product 2 then system checks the ATP of product 1.
Rule control is defined in the RBA rule. You can see this in /SAPAPO/RBA04 transaction.
Babu Kilari