Simply the people that say they may be your leader. The Commander rules just permit you to pick Legendary Creatures, typically. Those Planeswalkers tend to be an exception since they say they truly are, and because golden guideline 101.1 says they are able to do that:
101.1. Whenever a card's text directly contradicts these rules, the card takes precedence. The card overrides only the guideline that applies to that specific situation. The only real exemption is a new player can concede the video game anytime (see guideline 104.3a).
That said, Commander is intended to be a structure played casually for enjoyable with buddies — the official EDH site's Philosophy section says the maximum amount of. That exact same area explicitly promotes visitors to make exceptions or changes to your rules for local play if it is enjoyable. Until you're in certain sorts of formal sanctioned occasion, adherence towards Commander principles is only required insofaras your friends think it is fun. In the event that you along with your friends would you like to enable Planeswalkers as commanders, by all means, talk about it and create such a rule between yourselves, and then make a deck with a Planeswalker as a commander.
There is two resources of Commander guidelines you should check to verify the absence of Planeswalkers overall being permitted:
- People must select a popular creature because the "Commander" for their deck.
No reference to Planeswalkers.
- 903.1. Into the Commander variant, each deck is led by a celebrated animal designated as that deck’s commander. The Commander variation is made and popularized by fans; an independent principles committee keeps additional sources at MTGCommander.net. The Commander variant utilizes all of the regular principles for a Magic game, using after additions.