It's a new feature to 4.3. It's under View | Buffer Sets. You click on either the "View Scope" or "EditPane Scope" buffer sets checkbox.
When you open a new View, it's a separate window instance. It has a buffer set and an edit pane. With more than one View open, you can decide to share buffer sets (global scope) or each keep their own (View scope). When you split the View, you add another Edit pane. Each can use the View's buffer set, or have their own (EditPane scope)
Global scope: All Views and EditPanes share a common buffer set
View Scope: EditPanes in the same view share the same buffer set
EditPane Scope: Each EditPane can have it's own independent buffer set.
The editpane scope works when you have split the view (view | splitting). You can look at files side-by-side, and each side can have their own buffer set. Very useful in comparing files in two different directories (one set per side), for example.