Users become confused when adding mount points in the Custom partitioning UI. The UI presents a modal dialog with fields for path to mountpoint, and size of the implied volume. Of course Btrfs subvolumes have no size. But the field willing accepts a size without error or effect. Further, some user expect this is enabling and setting a quota. In any case, all expectations are set wrong by showing this field. Ideally, make it non-visible. A distant second might be to gray it out.