Allocated Size - This is the current allocated size of the datafile(s) in the database. Oracle uses blocks to extend the datafile(s) growth when demanded. The allocated size indicates the current size against the total allocated.
Total Size - The total size allocated to the datafile(s). Max Size column from the Oracle database is used if set.
Allocated Size Used % - is calculated based on the current allocated size of the datafile(s) against the total allocated size.
Refer to the attached document for detailed explanation.