light vs heavy swords?

[Aemilius Paulus]

It's not "swords", it's "weapon type". To answer the question, nothing.

Yeah, it is merely a designation that helps AI know what to recruit. Certain factions are scripted to recruit certain units AFAIK. That "balanced caesar" or "builder stalin", etc AI personalities thing in desc_strat may influence AI recruitment.

EDIT: oh, you meant the weapons? Hmm, beats me. I do know vanilla blacksmith buildings would first upgrade the light weapons, and then heavy. In EB, there is only one level of a blacksmith.