The I vs D thingie at AA is truly a relic of the past and needs to get with the times.. No other airline has this distinction.

To give you a contorted example. DCA has both the 737 and the Airbus.

Mexico and Canada are considered domestic (although Mexico pays international override.)
The Caribbean is considered international.

The Airbus is only international, like PHX. So they can fly everywhere including overwater.
The 737 is only domestic, so we can’t do any kind of Caribbean island flying - only Mexico and Canada.
But wait. We can’t fly into Mexico using any kind of overwater routes (greater than 162NM from shore.)
So we can fly DFW-MEX (that’s domestic) but not MIA-MEX (that’s international.)
We can fly DFW-SJD (domestic) but not DFW-CUN (international.)

Clear as mud? LOL