These are special phones just for blackberry developer to test their apps. Its a more limited spec than the official finalize Verison of the phones They are useful to test out bugs and fix error in apps. You can get one by attending blackberry dev events or winning from port a thon events.
There are 2 versions of Dev Alpha devices with slightly different configuration. Both have 1 GB RAM as opposed to 2GB in Z10. They were provided before the launch of Z10 so that developers can test their apps.