What the different tab lock states do is described at the bottom of the manual page on folder tabs:
https://www.gpsoft.com.au/help/opus12/index.html#!Documents/Tabs.htm
None of the lock states prevent tabs from being closed, but there is a script for that: Acecool Proper Tabs - Prevent closing locked tabs
(We might make something built-in in the future, since a lot of people seem to want it.)