What is Assignable,Rentable,Usable and Vacant significance and how Vacant field gets updated

Hi Folks,

Can anyone clarify me the below points

Example:

1) Own building and i need to define it and it doesn't have any floors. :

Building - with 1000 SFT

For this i have defined a building and provided gross areas as 1000 SFT and Floor as Ground floor and office with Assignable area as 1000 SFT.

As this is the own building and gross area is same as Assignable area 1000 SFT.

I can see vacant area also as 1000 SFT. Ideally it should not show as vacant as there is no vacant area.I have already updated the space assignment to an employee,after assigning also vacant area in building shows vacant as 1000 SFT.