function codes in BADI

HI,

I am learning BADI's. while going thru a document i got strucked with this can anyone explain me and how to do this.

1)"*Function codes can be entered for a BADI *for the corresponding menu entries in the user interface definition"

2) "Subscreen areas can be entered for a BADI. Screen enhancements can be implemented for these in the form of subscreen screens".