The removable version allows you to temporarily block the mouth of the skimmer so that the skimmer can be emptied without lowering the pond's water level. The Block Plate is not gasketed, so it won't indefinitely keep out all water, but it is a quick and easy solution for working inside the skimmer when the water is cold or disturbs visibility. Keep a Removable Block Plate on hand for those situations where it makes your work that much easier!
The "permanent" version is meant to be installed directly onto a helix skimmer body with lap sealant so that it creates a watertight seal. This allows you to use a skimmer body for creative puposes like a remove holding tank.