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, 28 March 2011
The Art of Ninjutsu Kata • Ninja Limited: Put this card into play in your home. Discard any other Kata cards you control. While this card remains in play, your Ninja Personalities and Followers have Cavalry, and during Cavalry Maneuvers, their units may only be assigned to battlefields with enemy units.
Two similar questions: 1) If have a ninja personality who is granted cavalry by a different card (say Straw Horse, or has cavalry printed on his card) while the Art of Ninjutsu is in play can he still assign only to battlefields with enemy units during Cavalry Maneuvers?
2) If my ninjas are unopposed at a battlefield, can my opponent play Snow-Swept Summit while the Art of Ninjutsu is in play? Would it matter which Maneuvers my ninjas assigned in (i.e. assigned as infantry or cavalry)?
Snow-Swept Summit Winter • Terrain Battle: You may take this action even if you control no units at the current battlefield while any Cavalry units are there: Before this battle's resolution, reduce the Force of each Personality and Follower at the current battlefield to 3.
Zen Faulkes (rules official), 28 March 2011
The Art of Ninjutsu: If have a ninja personality who is granted cavalry by a different card... while the Art of Ninjutsu is in play can he still assign only to battlefields with enemy units during Cavalry Maneuvers?
The limitation from The Art of Ninjutsu still applies. The limit does not depend on the previous effects.
If my ninjas are unopposed at a battlefield, can my opponent play Snow-Swept Summit while the Art of Ninjutsu is in play?
Yes. Snow-Swept Summit does not look for "base" or printed Cavalry; it doesn't matter how the card got the keyword.
Would it matter which Maneuvers my ninjas assigned in (i.e. assigned as infantry or cavalry)?
Not at all. The text is looking for the keyword "Cavalry", not when the cards assigned.