hello all, i was hoping someone would be able to explain teh basic theory behind sus chords and what they are used for. Alls i know is they sound good, especially if i follow them up with a maj7 chord ( i played Cmaj7, A7sus4 and it sounded pretty good). Also what kind of scales do they go with?

if any part of the question doesnt make sense please let me know. I dont know how else to phrase my question.

thanks