The 100GB usage quota limit is the upper limit for every Exchange Online user. Please delete outdated message to free up the storage. 

In case your mailbox usage has reached 90GB or above and you cannot be free up storage by deleting obsolete messages, please send email to cchelp@ust.hk to request for an Online Archive Mailbox (up to 1.5TB) with your account. Please be aware that once the Online Archive Mailbox is enabled, you cannot disable it. 

After a user's archive mailbox is enabled, messages and calendar items older than 2 years will be moved to your Online Archive mailbox (with same folders structure of your mailbox). You can access to both your mailbox and the Online Archive mailbox using Outlook and Outlook on the web but not with Outlook for mobiles. To search for messages older the two years, you will need to search the Online Archive mailbox instead of your mailbox.

Initially, 100 GB of additional storage is available, and it will auto-expand when it reached the 100GB capacity and so on. Auto-expanding archiving is only supported for mailboxes used for individual users or shared mailboxes with a growth rate that doesn't exceed 1 GB per day. Using journaling, transport rules, or auto-forwarding rules to copy messages to an archive mailbox for the purposes of archiving is not permitted.