Just to add in more information to this answer @M1Aston, as it’s not as complete/good as it could be.
The arc parameter above all else is a hint, used either when country auto-detection is incorrect, or when you want to override it.
By default, pages we think are targeted towards a person’s detected or selected country are given a small boost; this can be turned off or modified with the rbb parameter, to illustrate this:
https://www.mojeek.com/search?q=TERM&rbb=0
= A search with this boost turned off completely
https://www.mojeek.com/search?q=TERM&rbb=20
= A search with this boost as it is by default
https://www.mojeek.com/search?q=TERM&rbb=100
= A search with this boost as high as it can possibly go
To restrict results to a specific country and not just use a boost, reg is the correct parameter to use. This will include pages with the right country TLD, but also pages which don’t have that, but we think are targeted towards that country.
This being said, what country a page is targeted towards is not accurate and is something we’re actively trying to improve.