Oftentimes we will remove a sheet from the sheet index, and add it back in later. Currently, when you add a sheet to the sheet index, it "brands" that sheet model to the sheet index. In order to remove it, we have to remove it from the sheet index itself and then go into each individual sheet model, right click on the sheet model, and select remove from sheet index. This can be a very time consuming process and is not apparent to users. If we don't go to each sheet model and say remove from index, it will not be visible/selectable when we go to add it back into the sheet index later. It would be really nice if you could remove that "lock" on the sheet model that has been added to a sheet index.