Admin options for extensions are bad/misleading

I guess, we all know what enable and disable mean. However there is no install option or anything similar. So does this mean that enable == install and disable == uninstall?
Also what exactly does purge do?

I know, that purge is uninstall. But this needs clearification.
basically the options in the UI should be:

• install
• disable (db changes is retained)
• uninstall (db changes are removed)