I have one of the ID-51A Anniversary model HTs. It's a nice radio, fairly compact and built pretty well. It lacks direct keypad entry, but if I didn't use the APRS and full-duplex features on my TH-D72A the ID-51 would be my main HT. It still gets carried around for general use at least as much.
One thing I discovered with it is that in DR mode, if you select "Link to Reflector" the radio only gives you the option to select REF reflectors. There is no built in method of connecting to the other styles, the DCS or XRF (DExtra) reflectors and similar. This appears to be a similar issue with the ID-4100 and ID-5100, and I presume the ID-31. A brief look around suggests that the IC-7100 and IC-9100 get around this by requiring the full input of the linking/unlinking commands - though the IC-7100 seems to expect the same method of entry for all commands that we can use on the ID-51 and others.
I asked on the DSTAR Users group on Facebook and found a video that describes how to work around this limitation. The short story is, you enter DR mode and select your repeater (or hotspot). Click the center blue button on the To: field to bring up the SELECT menu, and the you choose the "Your Call Sign" option from that menu. You hit the quick menu button, and select "Add." Enter a descriptive name in the Name: field, and in the callsign field you enter the link command to link to the reflector. A reflector link command is the reflector ID followed by "L" - so if you're trying to link to XRF555A, in the Call Sign field you would put "XRF555AL" After you select this, hit the <<Add Write>> menu option to save it.
Now whenever you want to link to that reflector, you can just go to To: -> Your Call Sign -> Reflector ID. It'll send the Link command and you'll be all set.
This is much simpler than trying to program in a pile of different memories to do this for you, and you can enter new reflectors on the fly without requiring a PC.