For Yone's errata, what is considered an uncontrolled battlefield?

An uncontrolled battlefield is a battlefield that is not currently controlled by any player.

According to Rule 184.2.b, a battlefield is either "Controlled by a specific player or Controlled by no one." When a battlefield is "Controlled by no one," it is considered uncontrolled.

In the context of Yone's ability, "When I conquer a battlefield that was uncontrolled," this refers to a situation where you move Yone to a battlefield that has no controller at the moment the move occurs (or at the moment the showdown begins), and you subsequently establish control over it by the end of that showdown.

Key points regarding control and uncontrolled battlefields:

• Establishment of Control: Control is established by having units at a battlefield at the end of a showdown or combat (Rule 184.4).
• Losing Control: A player loses control of a battlefield immediately if they have no units there, unless the battlefield is currently contested (Rule 184.4.c).
• Contested Status: If you move a unit to an uncontrolled battlefield, it becomes contested (Rule 184.3.a). The battlefield remains contested until control is established or re-established (Rule 184.3.b).

If you move Yone to a battlefield that no player currently controls, it is an uncontrolled battlefield. If you then win the resulting showdown or combat and establish control, Yone's ability will trigger.