I discovered something about this, after accidentally selecting a more advanced unit on his list:
The units that the general requested appear at the settlement closest to him that is capable of producing that unit. So, one reason a general's unit construction list is so much larger than that of any settlement is that he can recruit from (potentially) all settlements. And the "1/3" etc. numbers on the units he recruits give some feedback on this: the 1st number is the build time, the 2nd number is the travel time, in turns.
I agree it's a great convenience, but the other way seems more "realistic."
I also wonder why reinforcing naval units takes 1 turn (although, they have to stay in the port), while reinforcing land units takes 2 turns.
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