This is a list of all the existing attributes you can assign to units. Please note this is just a list of attributes there are other options that can impact units that are not capabilities like training, morale etc. All of these are covered in more detail in the main EDU - export_descr_unit page.
can_sap- allows units to use sapping pointscan_withdraw- unused, I think?can_formed_charge- This does something but I'm not entirely sure whatcan_feign_rout- Fairly certain this is a cut special abilitycan_run_amok- unit will go out of control when scared enoughcan_swim- unit can swimcan_horde- unit is a horde unit, and will be disbanded if a horde army decides to settlehide_forest- stealth attributeshide_improved_foresthide_long_grasshide_anywheresea_faring- unit can board shipscommand- unit has an eagle and boosts morale around itheavy- unusedhardy- -2 fatigue modifiervery_hardy- -4 fatigue modifierextremely_hardy- -8 fatigue modifier (all 3 of these stack)inexhaustible- unaffected by fatigueinfinite_ammo- ammo does not depletenon_scaling- unit size unaffected by unit scalepower_charge- unit keeps charge bonus for longerslave- unusedfrighten_foot- demoralises nearby infantry units (not ones with javelins, though)frighten_mounted- frightens nearby cavalry and elephant unitsmercenary_unit- is a mercenary unitno_custom- stops units showing up in custom battlesis_peasant- unit counts for half it's size when reducing unrest, and is less likely to be added to rebel garrisonscantabrian_circle- adds cantabrian circle abilitydruid- adds druid chant abilityscreeching_women- adds screech abilitywarcry- adds warcry abilitygeneral_unit- unit can be used by generalsgeneral_unit_upgrade- unit can be used by generals, but only after a major event is activelegionary_name- unit will generate a legionary name