How system decides to create planned order / purchase reqsn for a material

Hi,

I know that in IMG-->Define creation indicator we can define for plant MRP group combination you define if the system should create Planned orders/ Pur reqsn / Planned orders which can be converted to Pur reqsn... but...

QUERY: Mrp group is not a mandatory field. if it is not entered how will the system decide to create pld ordr or pur reqsn? f1 help on mrp group in matl master says system will use matl type if mrp grp is not defined. but couldnu2019t identify where it is defined at material type (not there in matl type attrib defn). is it an internal defn for material type?

Thanks in advance,

Ashok