For customers with multi-domain plans, your control panel account is allocated a fixed amount of disk space and transfer equal to the amounts included in your hosting plan.  When you add a domain, you specify disk and transfer limits for that domain (these are preallocated for you if you use the "domain template" function in Plesk).  During the setup process (or under the SETUP) icon, you will also notice a DISK QUOTA value. 

Disk limits and disk quota are similar, but slightly different.  Disk limits is simply used for a reporting mechanism.  I.e., if you use more than XMB assigned to that domain, it triggers in a plesk usage report which you can see through the control panel, and of course, our account management team is notified as well.   Disk quota is actually an enforcement tool... this value actually sets a "quota" on your FTP user which limits you from actually putting more than XMB on the server.  By practice, we keep the disk limits and disk quota the same... what the domain is allocated is what is enforced.

Customers do not have access to the disk quota value.   3Essentials has disabled customer access to this intentionally due to a flaw in Plesk's implementation of this feature.  If you need this value changed, please submit a support ticket and we will change it for you.  We will only set this value to the same value as the "disk limit" for the domain.