RI stands for "Rest Interval".
RT stands for "Rest Time"
RI example:
8 x 50m on :15 RI
On your first 50m, if your time was :35, then with 15 seconds RI your interval for the set is :50, so for the duration of the set you are going to hold an interval of :55 even if you fatigue and begin swimming slower. So if on #6 you swim the 50m in :40 sec, your interval is still :50 sec based on your first 50m.
RT Example:
8 x 50m on :15 RT
No matter what time you swim, you will always take 15 seconds rest. If you swim the first 50m in :35 sec, with :15 RP you will push off again when the clock hits :50 sec. If on #6 you swim in :45 sec, you still take 15 seconds rest so you will leave again when the clock hits :55 sec.