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, 13 May 2008
Unexpected Reinforcements 3 Gold Tactical Battle: Target one of your Tacticians: Create three 1F/1C/1PH Samurai Personalities with your Clan alignment at the current battlefield. Remove them from the game after the battle ends. 3 Focus
We've a local Lion player who insists that he can create the three 1/1 Personalities in the opposing army. So he will attack with some guys at a province, play this once or twice, kill them all and gain a ton of honor. Or, when you attack him he'll defend somewhere where you don't attack, and do the same thing, going over 40 and winning on his turn.
Can you do this or can you only create the units in your army?
Alex Jones (rules official), 13 May 2008
A player other than the Attacker who was not invited by the Attacker can not have his or her units assign to the attacking side, move into the attacking side, or be brought into play on the attacking side during a battle. This prohibition includes the Defender and third-party players.
Likewise, a player other than the Defender who was not invited by the Defender can not have his or her units assign to the defending side, move into the defending side, or be brought into play on the defending side during a battle. This prohibition includes the Attacker and third-party players.
You can only create the units in your army.