The iOS version is different to the Android version, and JRemote2 is different to the original JRemote for Android.
As to the original question, JRemote2 has a PlayMode setting, which is accessible through the PlayMode icon, which isn't always visible, but with a phone held in landscape mode will show up. It is the "1+" icon shown in the first attached image.
If you tap on that PlayMode icon you can change it. The options are shown in the second attached image.
As far as I can tell, what the PlayMode does is change the behaviour when you tap a single Track in the screen shown in the first two images.
If it is set to "Play Now" JRemote2 just plays the Track that was tapped.
If it is set to "Play All Now" JRemote2 adds all Tracks from the Album to Playing Now, and plays from the Track that was tapped to the last Track.
Also, the PlayMode has no effect on what JRemote2 does when the Play button shown in the middle of the first image (let's call that the Album Detail screen) is tapped. JRemote2 always clears Playing Now, adds all Tracks from the Album to Playing Now, and starts playback from the first Track. I actually expected this button's behaviour to be affected by the PlayMode in selected, but I guess that wouldn't make sense, as JRemote2 would either always play just the first Track, or play the whole Album from the first Track. So it makes more sense that the PlayMode only applies when you tap just one Track.
There are also long press actions in JRemote2.
Long press on an Album Cover and the options shown in the third attached image are displayed.
Long press on a single Track and the options shown in the fourth attached image are displayed.
I think those options are self-explanatory, but note that "Play Now" clears Playing Now, adds the Track to Playing Now and plays it, while "Play Next" and "Add to 'Playing Now' list" just update the Playing Now list as expected.
EDIT:
Actually, the "Play Next" option inserts the Track as the next to play if JRemote2 is currently playing. However, if it isn't playing, "Play Next" just appends the Track to Playing Now. I don't think that is what was intended.
Also, the "Add to playlist" option doesn't seem to do anything at all, that I can see.
I'm not sure if the above is exactly how JRemote2 was intended to work, particularly for the Play button in the middle of the Album Detail screen, but that is how it works now.
Some of this was discussed starting at this post here:
https://yabb.jriver.com/interact/index.php/topic,125322.msg868060.html#msg868060
