The MGN09120 is a D0 gauge accessory for fuse-switch disconnectors in the D02 size, rated 20 A. It is colour-coded Blue, which on the D0 line indicates the current rating — so at a glance on the panel, the blue body tells you this is the 20 A insert.
This is an installation accessory — a gauge insert that fits into the fuse-switch disconnector base. The 20 A rating matches the fuse it accommodates, so when you are replacing or specifying a D02 fuse holder, the gauge is what ensures the correct fuse size is used. No wiring or panel modification needed; it snaps into the existing disconnector.