Interesting question -- I haven't actually used Bob Puff's Disk Communicator in years. DISKCOM3 was one of a series of disk to file utilities whose purpose was to allow sending a disk (image) via modem. It was probably the last and arguably the best of the group that included SCRUNCH (the first that I know of), SHRINK, SQUISH, CRUSHER, and MASHER. DISCOMM also did some slight compression, but I believe it was limited to "compressing" empty sectors. DISCOMM was intended for floppies up to 1 MB in size. (Remember that 1 MB was a BIG file to be sent via modem when these utilities were written.)
ATR's had a little different intended purpose -- to support the SIO2PC system. "Proper" ATR images never use compression. They *can be* made into compressed ATR's, but that sometimes causes problems for the user. They are real disk images up to 16 MB (maybe more), and have an attached group of index bytes (16, I think) that provide relevant info about the disk image. Since APE can use ATR, DCM, and XFD images there is no real need to convert DISKCOMM images, but several utilities including Atarimax's IMAGIC allow conversions.
Here's a website that has more info about DISKCOMM:
http://home.planet.nl/~ernest/atarixle.html -Larry