Action name: MakeUnknownThis command deletes the current instruction or data, converting it to 'unexplored' bytes. IDA will delete the subsequent instructions if there are no more references to them (functions are never deleted).
If you have selected a range using the anchor, all the bytes in this range will be converted into 'unexplored' bytes. In this case, IDA will not delete any other instructions even if there are no references to them after the deletion.
See also Edit submenu