This is new feature for SQL Server 2014. We can use SSD to extend the buffer pool size. The idea is using the faster SSD , in the event when the physical memory need more space, it will use SSD instead of physical disk. However, for testing purpose, the SQL server can use any disk as extension, it does not limit to SSD only. Database engine would not have idea whether it is SSD or not.
The size of the Buffer pool extension must be large than current MAX server memory otherwise, we will get the error when configure the buffer pool extension