Chastity is usually a test of will, because it can be taken off easily even without a key. But this tag is for truly inescapable ones: fixed by piercings, welded into place, secured by long urethral rods etc.
It's similar to permanent_chastity, but permanent_chastity is about breaking a key, or gluing the lock on an already easily removable cage. It's kinky, but not real most of the time. And one more crucial difference: inescapable chastity isn't permanent, it can be taken off, but only by a keyholder/by getting access to the key/destroying the cage itself.
It's explicitly "inescapable_chastity_cage" because chastity belts are already created inescapable most of the time.