M4A is AAC in Apple's favorite outfit: the audio-only MP4 your iPhone speaks natively. It was born in 2003, popularized by Apple's iTunes Store, and its real weak spot is older Windows software and stubborn car stereos that expect a .mp3.
Video is the heaviest thing most of us ever attach: screen recordings, camera clips, whole talks. Stands in for any video format Flippy can read: MP4, MOV, MKV, WEBM and the rest. That makes it the right destination when what you need is any moving picture: recordings, rips, exports, downloads.