What is the use of different type os Exchange Rate Type ?

Hi All

While defining Exchange rate type in ORMS below are the options listed:

1) Consolidation ( C)

2) Operational  (O)

3) Purchase Order (P)

4) Letter of credit/Bank (L)

5) Customs entry (U)

6) Transportation

7) Logistics (G) .

I understand the usage of first three i.e consolidated ,operational & Purchase order types  . But what does other exchange rate  types  offer .

What kind of  transactions does these other exchange types are  being used ?

Kindly suggest .

Thanks in advance !

Thanks

Chunnal