Oh, that enables/disables that field until the user has meet the requirements you set there.
For example (this does not apply to moderators or administrators, the fields are always available for them)
Always off - means the field is never available to fill out
Off for registration - means they can fill it in after they register, but not during
Always on - means they can always fill the field in, including during registration
Set according to post count - requires the users to have at least the number of posts you set in the post count field before the field is able to be filled in
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