Domain disk space indicates how much disk space is used by the entire domain. It is calculated by the Plesk statistics utility and includes the amount of disk space taken up by webpages, mail accounts, databases, and other domain data. You select object types that Panel will consider when calculating disk space usage on Tools & Settings> Server Settings page in Plesk control panel. If a domain exceeds the "Domain disk space" limit, both the server administrator and the domain owner receive notifications about it.
The following links may be helpful regarding disk space: How disk usage is calculated by Plesk
Hard disk quota
This is a file system feature that limits how much disk space can be taken up by files that are owned by a specific system account (either a domain FTP user, webuser, or subdomain FTP user). This file system quota cannot be exceeded, and if the limit is reached, the system user will not be able to create files anymore and the error "no more space left on device" will appear.
Note: (Linux hosting) Confirm that your operating system supports hard disk quota before you set any value other than Unlimited. In case you define a hard quota when it is not supported, you will get a synchronization conflict on all the plan's subscriptions /768
To adjust hard quotas, allow permission "Hard disk quota assignment". The parameter can be found in:
Service Plans > select a plan > Permissions tab Subscriptions > select a subscription > Customize > Permissions tab
Then go to Subscriptions > select a subscription > Websites & Domains -> Web Hosting Access type the desired value in megabytes into the Hard disk quota box.
You can check hard quota for the system user using the command:
repquota -as | grep <username> testuser -- 488 0 10000M 94 0 0
For Windows see the article Managing Disk Quotas