I am curious about Teferi, Mage of Zhalfir. Does his second ability allow me to cast creatures outside the game that I own as well? Such as cards in my binder? Or only creatures that I'm using in the game?
Teferi's ability applies to all "[c]reature cards you own that aren't on the battlefield"; the ability applies to such cards outside the game, not just cards in any of the game's zones.
So I can legally cast any creature in my collection of cards?
You can legally cast any card you could already legally cast (i.e. - in your deck). It just gives those cards Flash.
No, this does not allow you to cast things that are not part of the game (like from your binder). He does not have a line that read "You may cast creatures that you own from outside the current game."
As Eli Shiffrin clarified to me, cards "that aren't on the battlefield" do not include cards outside the game. Under C.R. 400.11c, this is subject to certain exceptions not applicable to Teferi's second ability.
EDIT (Mar. 11, 2021): One rule was renumbered in the meantime. Note also that in the meantime, Eli Shiffrin deleted his Twitter account, so his ruling no longer appears at the linked Tweet.
Teferi's ability applies to all "[c]reature cards you own that aren't on the battlefield"; the ability applies to such cards outside the game, not just cards in any of the game's zones.EDIT: See comment 6.
No, this does not allow you to cast things that are not part of the game (like from your binder). He does not have a line that read "You may cast creatures that you own from outside the current game."
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EDIT (Mar. 11, 2021): One rule was renumbered in the meantime. Note also that in the meantime, Eli Shiffrin deleted his Twitter account, so his ruling no longer appears at the linked Tweet.