Explicit call business rules are ones that execute not based on some event such as a row being inserted or updated, but rather these need to be intentionally executed from within code.  There are several of these business rules that are part of the baseline solution, but they could also be used as part of a customized solution.  In those cases, they would need to be explicitly executed from the customized code (e.g. an over-ridden policy or some custom mpm). 

Thanks for your reply,

One sample document for customized Explicit business rule will be very helpful.