"Key" seems to mean quite a few different things. Good catch, <s>it would involve a lot of manual retagging though</s> doesn't matter anymore though :P

Let's see:

### Mechanical key

The stuff a keyboard is composed of. Mostly in combination with [tag:keyboard], [tag:keyboard-shortcut] or [tag:keyboard-layout].

### Registry Key

The ones used in Windows Registry. Mostly in combination with [tag:registry].

### SSH Key

An SSH key generated by `ssh-keygen` tool. Mostly in combination with [tag:ssh].