I just did a network diagram and calculated ES, EF, LS, LF and float for each activity. My sample question says to find TF, FF, INTF, and IDF for each activity.

I understand that
TF = Total Float = LS - ES
FF = Free Float = ES of one task - LS of previous task

But I don't know what the other two mean.
INTF - what is this? and how to calculate it?
IDF - what is this? and how to calculate it?