Lets say i went in battle with 3 spear militias, 3 lower militias and 3 pavic crosbowman (venice).
Everyone suffered some light causalties so i have 9 wounded units. I retrurn to city (with 3 recruitment slots).
If i simly strated retraining, it would took 3 turns (9 units).

But if i merge wounded units, i will get 2 healthy and one almost depleated for each unit type. So i have 3 almost depleated units and can retrain them in one turn.
Maybe its common knowladge, but i didn't found it anywhere and it realy speeds up recovary after battle.

Btw. if i retrain unit, does newcomers starts with zero experience (so exp. will get averaged and fall), or they gain same rank as retrained unit?