Purchasing value key determines the following things:
1) Reminders keys- There are 3 keys which determine in how many days before or after the vendor has to be reminded or urging letters for the material procurement.
2) Over and under deliveries tolerances.
3) Order acknowledgement requirements in PO.
It gives all the conditions in which the procurement is made. You can have different purchasing value keys which have to be customized with below path:
SPRO-IMG – MM – Purchasing – MM – Define Purchasing Value Keys or Transaction Code OME1. Screen shots are given below