AIFF vs AIFF-c

I was always able to import wave and AIFF files without converting into a file matched session in pro tools, now AIFF-c files need to convert to AIFF? Can anyone fill me in on the diff? The C is supposed to stand for compression, but as i understand it, it's just for meta data or some shit, and has no impact on the quality of the sample. I really just want to add and not convert??

thanx
T