They both use magnets and both work with Versa58 scales, but they go on different things and solve different problems.
MagMount plates attach to scales. When two scales each have a MagMount plate of opposite polarity, they snap together into a compact standalone tool - no Swiss Army Knife needed. One plate includes a loop for keychain, carabiner, or bag attachment. This is the "use Versa58 without the knife" option: a Pocket Flare paired with a Pen Scale on your keychain, a USB-C Flash Drive paired with a light in your bag, or whatever combination fits your day.
MagDock plates attach to the knife. Each MagDock plate has a built-in Interface Plate that mechanically connects to your Swiss Army Knife, just like a standard scale. Once a MagDock plate is on the knife, any scale with a MagMount plate of opposite polarity snaps on magnetically instead of mechanically - pop the Pocket Flare off to use as a work light, plug in the USB-C drive, then snap the scale back on when you're done.
Used together, the connection chain is: Swiss Army Knife → MagDock (mechanical) → MagMount on scale (magnetic) → Scale. You get magnetic quick-swap on the knife and standalone pairing off it.
Easy way to tell them apart: MagDock plates have a built-in Interface Plate on one side. MagMount plates do not. Both are laser-engraved with their name and a + or - polarity marking - a + plate connects to a - plate.