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, 24 March 2014
My playgroup found the wording on Mirumoto Nokkai confusing:
Nokkai has +1F/+1C while dueling, or opposed by, a Phoenix Clan Personality.
Does this mean Nokkai has +1F/+1C [while dueling and opposed by a Phoenix Clan Personality]?
Does it mean Nokkai has +1F/+1C [while dueling] or [opposed by a Phoenix Clan Personality]?
I find the Mirumoto Tsuge version to be easier to understand:
Tsuge gains +1F/+1C while facing one or more Phoenix Clan Personalities in a battle or duel.
The confusion may stem from the usage of "OR": In ordinary language, outside of contexts such as formal logic, mathematics and programming, "or" sometimes has the meaning of "exclusive or". For example, "Please ring me or send an email" likely means "do one or the other, but not both". On the other hand, "Her grades are so good that she's either very bright or studies hard" allows for the possibility that the person is both bright and works hard. In other words, in ordinary language "or" can mean inclusive or exclusive or. Usually the intended meaning is clear from the context.
A player, 26 March 2014
We tend to avoid programmer language because only a small fraction of our players are programmers; instead we use regular English. When we use "or" for conditions, therefore it's never exclusive. You can fulfil the condition by either, or by both clauses. Note that this is so because we're talking about checking conditions, not fulfilling instructions, which are wildly different.
Nokkai's trait should be interpreted as "Has +1F/+1C while he's dueling a Phoenix Clan Personality, or while opposed by a Phoenix Clan Personality in battle, or by both conditions"
PS The example you provide is for exclusive instructions ("ring me or send me an email"). Many cards will give you that kind of exclusive instructions (for example, Ring of Water, allows you to chose between two exclusive effects separated by an exclusive "or"). You choose between one of the subsets of effects. Conditions work usually inclusively (unless an exclusive conjuction is used, such as "either"), even in regular English. "You have a higher risk of heart disease if you have overweight or you smoke" means that you will also have a higher risk of heart disease if you have overweight AND you also smoke.