One is an integer and the the is an object ID, other than that difference no. The meaning behind the id is the same, to reference a single entry by an absolute, non changeable value.

So, if I am modelling this, they are synonymous.

They provide the same information, i.e. a pointer to a source episode or movie

Correct and they are named differently because they don't provide the same value (integer vs. a BSON Object Id). Credits work way differently for TV and it didn't make sense to model them the same way. In fact, one day it would be really nice to mimic the model back to movies but that's another job for another time ;)

Great, thanks. For my purposes, I'll store them the same until you change it ;)