A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 21 October 2010
This is a question about an older CE legal card, so i'm sorry if this question has been addressed, I used the search function on this board, as well as checked the rules.l5r.com site and couldn't find anything.
Moto Kang's Sword Text Weapon • Unique Attaches to a Unicorn Clan Personality paying 1 less Gold. Battle: If this card is opposed, destroy it: Put a target card from your Fate discard pile into your hand.
If Moto Kang's Sword is attached to a personality at a battlefield unopposed, can you use the battle action and not destroy the card?
To me the constraints block means the card must be destroyed if it's opposed, not that the action requires opposition and card destruction.
A player, 21 October 2010
If Moto Kang's Sword is attached to a personality at a battlefield unopposed, can you use the battle action and not destroy the card?
No. "If this card is opposed" is a requirement. Requirements come before costs. A constraints block that would match your description would be: "Destroy this card unless it is unopposed".