Some versions of the metformin extended release tablets require the use of a special cage to ensure they remain vertical for the dissolution test. The inserts are placed into the standard USP1 baskets, then the tablet is placed inside the insert. The insert is designed so that it can not topple or tilt during the test. Full details on this test can be found on the USP website.
There are different inserts for the 500mg and 750mg tablets.
VERTHLD-750 is the larger insert and is used with the 1000mg tablet and VERTHLD-500 is used with the smaller 500mg version.