Historical Rules · 1 October 2012
Rule 205.3j
Official source text from the 1 October 2012 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, Tamiyo, Tezzeret, Tibalt, Venser, and Vraska. 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.