PS substituation

Hi,

a) What is the fuction of exit, while creating the subtituations?

b) subtitution method can be used for below Scenarios

1. whenever the control key is PS02 (i.e external actvity) than Tracking number will be "EXPUR-PROJ"

2. same way whenever the material is assigned to internal activity, having procurment data tab page for material details.

if yes, what are the step for creating subtitution

else

any other solution

Thanx

Suresh