Explanation for important reserved space
Space Reserved is the size you setup using either:


SSMS = Database Properties / Files Database file / Initial Size (MB)
Alter Database [XXX] MODIFY FILE ( NAME = N'XXX', SIZE = ...KB)
Autogrowth changes it as well


You can check these setting using SSMS or this query:

SELECT file_id, name, type_desc, physical_name, size = size/128, max_size = max_size/128
FROM sys.database_files ;


Space Used is the size of your existing data pages in your data file. You can use this query:

SELECT name, groupid, filename, 
   Cast(size/128.0 AS DECIMAL(10,2)) AS [Allocated MB],
   Cast(FILEPROPERTY(name, 'SpaceUsed')/128.0 AS DECIMAL(10,2)) AS [Used MB],
   Cast([maxsize]/128.0 AS DECIMAL(10,2)) AS [Max Size MB],
   Cast([maxsize]/128.0-(FILEPROPERTY(name, 'SpaceUsed')/128.0) AS DECIMAL(10,2)) AS [Available Space in MB]
FROM sysfiles
ORDER BY groupid DESC