Like infantry-melee, infantry-polearm, infantry-ranged, artillery, cavalry-melee etc. The types would be defined in the tech tree xml and adjustments to variables could be edited there too. It'd be optional with untagged units getting a default value.
These tags could then influence:
AI, allowing better tactics (would it require having some editable AI in the tech tree XML or similar place??)
Attack/defense balancing (eg infantry x is good against cavalry but not so great against infantry-melee)
Other more sophisticated later uses:
Influence formations
Influence attack style, eg run-past attacks etc instead of stand and fire