Are you asking about the alternator ratings? That's right.
And rarely can we find anyone to give a full a proper answer about an alternator we're buying.
Whomever makes the alternator can rate the 'amperage' at whatever rpm and voltage they feel like.
Obviously most use a high rpm and low voltage to make their claim.
There's no standard.
This came up in a thread on speed-talk.https://www.speed-talk.com/forum/viewtopic.php?f=20&t=41740
My takeaway is that within reason, more windings can be used to increase the strength of the rotor's magnetic field, and if the stator is matched more power is possible.
Of course more windings in the same space mean more expense and also less room for cooling....
A longer space makes a little more room for windings and cooling.
Tighter tolerances in machining and assembly can bring field closer to the stator windings.
Changing the way its wired from Y to delta will change the performance curve.
Some combination of the above accounts for the better overall performance of the squarebacks, revised squarebacks and the Denso alternators that followed the 'roundbacks.'
But the problem for us, the consumer, is we don't know if the rebuilder or aftermarket seller has matched the parts, or just improved the top end at expense of the low speed, and/or fudged the results a litle by playing games with the test voltage etc.
A couple example I can show of a higher rated alt having a drop low rpm performance are from AC Delco's catalog.
These are GM alternators but its nice they actually used to show the info.
Pay attention to the RPM and the voltage!
Being a pseudo-engineering graph, we don't know if the 12 Volts was the real test voltage or the nominal (common language) description of the alternator.
In other words this is 12 Volt Delcotron vs a 6 Volt generator.
(GM hated calling their AC generators 'alternators' like Chrysler did.
The other thing is the RPMs are alternator shaft speed. Usually 2 to 2.5 times the crank speed depending on the pulley set.
Here's another model.
I'm familar with these two because AMC used them both in the Jeeps. The 15SI was the preferred heavy duty unit presumably because of its good low rpm perfromance.