Difference between provisioned, allocated and used storage

Summary:

We provisioned an ADW instance with 1 TB storage and imported some data. Now, the allocated storage = 626 GB, while used storage = 547 GB, which indicates that only 79 GB of storage is left, which is not sufficient for future data loads. Even when we provision a new ADB with 1 TB storage, there's a difference of only 2 GB between allocated and used storage, both of which are in the range of 43-48 GB.

I'm new to ADB, so wish to confirm if ADB storage is elastic. The customer does not have auto-scaling enabled, so we have to explicitly raise a request for scale up when needed.