What is define('VHOST', 'yes'); in wp-config?

Hello - I'm just going through the installation instructions for the Domain Mapping plugin and there is one image that shows a wp-config file (https://premium.wpmudev.org/project/domain-mapping/installation/).

In particular, I am wondering about:

define('VHOST', 'yes'); which is commented out on my install (// define('VHOST', 'yes'); ) - can someone explain what this is all about and why/when it would be uncommented out (as it is in the image used on the plugin page)?

As well, how can I check to ensure that my network is set up for subdomains (as opposed to sub-directories)? I didn't install this myself (wish I had) but I do have this in my wp-config:

define('SUBDOMAIN_INSTALL', true);

and when I add new sites, they appear to be working as I want - site1.example.com, site2.example.com

I'm just checking to make sure that everything will work as I expect when I go to use this plugin.

Cheers!
dale