TYPE OF IS to be selected

Hello all of you,

I want to know

What is the TYPE OF IS to be selected while creating an information structure for user defined LIS? There are three types given when you go to T.code MC 21

STANDARD

C Without period unit

T Transfer to SAP-BW

What does each of the type mean and what should be selected under what circumstances? Please explain.

Thank you,

Tristan.