You can have stuff on other tracks above or below, but there are a bunch of instances where you can’t RD or don’t want to, particularly if the other tracks have spaces in them, usually you can’t RD if a track above or below has a cut or join in the same area as the segment you are trying to RD or if a cut or join is after in time on another track the segment you you are trying to perform a RD on, You can ripple delete if there is a space in another track within the segment you are RD, be aware a ripple delete with spaces on other tracks will cascade till a track joins somewhere, its best to put 2 or 3 tracks of video and play with it till you see what it does, ^Z can be a life saver.