Unless you're looking for a more technically specific answer, RLA deals with compressor amp draw and FLA deals with fan motor amp draw. Measured RLA is usually around half of nameplate rating while FLA is usually near nameplate rating.
There is nothing more frightful than ignorance in action....Mark Twain
Here is a stab at your question. It is a good one and sometimes these terms are confusing. Hvac Pro brought up an interesting point about FLA being related to fan motors, but lets look at this as a compressor question. RLA is a rating point that manufactures use to help guide you in selecting a contactor for a compressor. It is a "Rating" indicator. FLA is a thing of the past, for compressor stamping, anyway, because it could be misconstrewed, unfortunately as RLA has become. "Full Load Amps" were an indicator of the motor at "Full Load". Know, I will doublespeak a little and say that this information is very crucial and needed in large centrifugal and screw plants that are using amperage feedback to tell the plant what kind of work is being done, but sticking to your standard 2 horse power "tin can" that information can be as misleading as helpful. That is to say, when a tech is troubleshooting and he clamps his amp meter on the leads and sees the unit pulling 1 amp above RLA or 2 amps less than FLA, he wants to condemn the motor. It is a bad practice that some get into.
A while back I noticed that Tecumseh had abandoned RLA and FLA all together and just posted LRA. I think that this was a great idea. In order to know WHAT AMPERAGE the compressor should be PULLING WHEN you need a curve chart. If you have the LRA (locked rotor) information, you can properly identify if you have good overload operation and if you are actually locked. You can also approximate what kind of high end amp draw you would see on a motor if it was loaded.
I hope this helps some.