Allocated Space

Hi

Does anyone can help me understand what does Allocated Space mean?

Searching for that in DSM pdf's page 308:

https://topics-cdn.dell.com/pdf/storage-sc2000_administrator-guide_en-us.pdf

Allocated Space - "Amount of disk space allocated on the disks" 

Wow......amazing information.

Anyone can please help me? because i have only 9.8TB used from 17.8TB capacity.

Someone stole 3TB from me (only 5.25TB is free)