This button tracks the state of the IMMUTABLE flag of the current cell, and will alter the flag if editing mode is changed. When the current cell is IMMUTABLE, it can not be modified, and all editing features are disabled. The side menu is hidden, the Modify Menu is disabled, and all but this button and Create Cell are disabled in the Edit Menu.
If there is no current cell, editing features are disabled.
Cells read into Xic through the library mechanism have the IMMUTABLE flag set. This button can be used to allow modification of these cells.
Setting the IMMUTABLE flag of the current cell from the Flags button in the Cells Listing panel from the Cells Menu or with the !setflag command will have the same effect as use of this button.