The training field has three options: Mounted, ranged and sparring.
Sparring is the most used option. In this mode, you'll fight with up to 4 of your men with up to 9 randomly picked among the remainder stand around to watch. Assuming you clear the spar, experience will be awarded to the men you've beaten and all of your remaining men. I don't know the specific algorithm behind the experience awarded, but these seems to be the case:
- The watchers (or the group containing the watchers) will gain more experience at the end of the exercise.
- Sparring with the highest-leveled and lowest-leveled of your men award the least experience to the watchers.
- Sparring with troops you supersede too much level-wise will not award much experience.
- Sparring with ranged units award NO or VERY LITTLE experience, same for cavalry - the latter may be caused by note #2.
There is a random element in it somewhere, I reckon.
Mounted and ranged training will award experience to your mounted and ranged troops respectively. The number may seem large at first sight compared to sparring, but keep in mind that cavalry and ranged troops are usually of medium to high tier already and won't benefit much from the field any more.
My experience is that the second tier (Swadian Militia, Vaegir/Nord/Sarranid Footmen, Rhodok Spearmen) are the most cost-effective trainees using the training ground. Any other troops won't benefit as much, and you'd be better off just sending them to the battlefield and hope for the best.
Or better still, invest in Training.