I purchased a reseller account and I noticed that I have access to web hosting features (File manager, mysql, etc.). Is that just to host custom webmail (i.e. roundcube) installations? I can’t seem to find anything about the web hosting features on here.
That is because there are no web hosting features offered here. The service MXroute provides is solely email.
You see these ‘features’ because MXroute is built on top of a web hosting control panel - however the web hosting portions have been stripped out where possible.
That was my understanding of the service, that’s why I’m trying to wrap my mind around this.
Prior to your reply last night, I was able to upload roundcube.zip via the file manager to a sub folder in the TLD’s main folder called “mail” that was auto generated after i created the subdomain and all that in direct admin. Then, I extracted the archive and went to the url - to my surprise, it actually seemed to work. That’s when I made this post, I didn’t do much testing with it.
So i guess my question is, can i do that? I obviously don’t want to break the TOS/AUP.
Hosting your own files isn’t allowed, and will be shut down if dicovered. If you want the webmail clients on your own url that’s already possible by using our roundcube, a bonus being that you don’t have to keep it up to date 
https://mxroutehelp.com/index.php/2019/08/25/custom-webmail-pop-imap-smtp-domain/
I mean, I guess I don’t care if you host some web data on reseller you’d just have to understand that my intent is not to be a web hosting service and that I’m not considering the wellbeing of any web hosting content when performing maintenance, backups, recoveries, upgrades, or further service improvements. I thought I disabled PHP for all resellers though.