Every 24 hours the system scans your FTP home directory, each of your mailboxes and databases to summarize disk space used by these resources which makes summary disk usage for the last day. Summary readings of each daily scan are added up and divided by days elapsed from the beginning of the billing month to make your average summary disk usage. If your hosting plan is configured with the summary disk usage resource, you will see both figures on your Account > Quick Access page:
Free disk space allocated for all your disk space consuming resources makes your initial summary disk usage limit. However, it doesn't stop you from using more disk space as quotas do. If you exceed it, you will be charged overlimit fees. To prevent it, increase your summary disk usage limit clicking the Edit icon next to the summary disk usage bar and entering a new summary disk usage limit. Summary disk usage limit can be max restricted which means you won't be able to increase it any higher when this limit is reached. To check how much you can increase your quotas to, go to Billing > Online price and check the Max field in the Summary Disk Usage entry.
Keep in mind that your hosting plan can be configured to suspend your account if you use your summary disk usage limit by certain percent. Before suspension, you would be usually warned by e-mail of approaching your limit.