The DriveInfo class has two similar properties:

They are similar, but not interchangeable as there is a subtle difference.

The main operating systems support a concept called a disk quota, which allows administrators to set the allowable amount of disk space that users can be allocated to a user. Windows, macOS & Linux/Unix all implement this in different ways.

The difference between these two properties is that the AvailableFreeSpace factors in disk quotas. This means that it is possible for the Drive to have space, but the user may have exhausted their quota. In which case AvailableFreeSpace is probably what you mean to use if the current user is intending to write some data.

TLDR

The AvailableFreeSpace property return the actual amount of space the current user can write to, factoring in disk space and user quotas.

Happy hacking!