Tolerances in Procurement process

Environment: No material Master

Procurement process: Pur Requisition -> Pur Order -> GR -> Invoice -> Payments

Please help me understanding the tolerances. Question at each stage is:

1) which is set at the PR, PO, GR, Inv

2) Which will default and from where (eg. this tolerance will default from PR in PO)

3) Compares which qty with which wty or which price with which price (eg. this tolerance tallies qty of PO with qty of GR at the time of Invoice)