Historical Rules · 16 July 2010
Rule 204.3i
Official source text from the 16 July 2010 Rules release
Planeswalkers have their own unique set of subtypes; these subtypes are called planeswalker types. The planeswalker types are Ajani, Bolas, Chandra, Elspeth, Garruk, Gideon, Jace, Liliana, Nissa, Sarkhan, Sorin, and Tezzeret. If two or more planeswalkers that share a planeswalker type are on the battlefield, all are put into their owners' graveyards. This "planeswalker uniqueness rule" is a state-based action. See rule 704.