So it looks like either of the 1st two, but I'm a bit confused which one as they are both:
This can be changed if you have multiple XenForo installations on the same domain.
So would I leave my my live sit config.php alone, and just add in the test forum config.php
EITHER
$config['cookie']['prefix'] - default: 'somethingelse_'
OR
$config['cookie']['path'] - default: '/nameofsubdirectory'
??
Thanks again