With the ALEX executable, the AI retrains its troops if they are understrength (and recruitable in the town the unit is stationed in.)

With the RTW and BI executables, the AI does not do any retraining, just builds new units.

Personally I don't like the AI retraining because it encourages the AI to spam armies and deplete its populations too fast.

So I use the BI executable.

The advantage of BI over RTW is much more naval invasions - the RTW exe hardly does any.