+If nil, all buffers are kept, up to `buffers-tab-max-size', in usual order.
+Otherwise, each function in the list must take arguments (BUF1 BUF2).
+BUF1 is the candidate, and BUF2 is the current buffer (first in the buffers
+list). The function should return non-nil if BUF1 should be added to the
+buffers tab. BUF1 will be omitted if any of the functions returns nil.
+
+Defaults to `select-buffers-tab-buffers-by-mode', which adds BUF1 if BUF1 and
+BUF2 have the same major mode, or both match `buffers-tab-grouping-regexp'."
+ :type '(repeat function)