Description of Ordo Cavalry Unique Tech says Cavalry regenerates HP in combat. I assumed it is for both melee and ranged and the siege cavalry as well. 
But turns out it affects only Melee Cavalry. This should be accurately mentioned in description.
- Cavalry = melee cav only
- Mounted units = melee cav, ranged cav, siege cav
Ordo does not affect ranged cav, siege cav (mounted trebs = siege cav)
From wiki:
Cavalry except Mounted Trebuchet regenerate 150% of max hit points per minute when attacking units
In this case:
Stable units regenerate 150% of max HP/min
(2% of max HP/s)