A lot of amp manufacturers use confusing ratings to either make their amps look better or worse, depending on their target market.

Here is a general rule as to how much total RMS the amp can theoretically put out in a car.

First of all in all these calculations I'll assume that the volatage is 13.8V, which is generally what the voltage is when the engine is running.

Another assumption will be that
Class A/B amps are 60% efficient
Class D amps are 80% efficient

The maximum RMS wattage that an amp can put is:

Voltage x Max Current Draw x Efficiency

For example if your amp has a 40 amp fuse and is class a/b it works out like this:

13.8 x 40 x 0.6 = 331 WRMS total output.

For an actual example we'll use the Directed 1100D..
It's a Class D amp (80% efficient) with a 90amp fuse..

13.8 x 90 x0.8 = 993WRMS total output.

So if you're ever wondering what a 1400W Legacy can put out in real terms. Consider the fact that it has a 40amp fuse..

The efficiency ratings are not always what I've stated. Often they are worse, but I've put them as they are to show a "best case" scenario..