Historical Rules · 1 May 2011
Rule 204.3i
Official source text from the 1 May 2011 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, Karn, Koth, Liliana, Nissa, Sarkhan, Sorin, Tezzeret, and Venser. 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.