ER-POE5 Automatic POE?

If I set one of my ports to 48v POE, does this just mean that it is capable of providing this current for a device that needs it and it will automatically detect the need or is this just full on?

If I plug a non-POE device to this port, will it try to provide POE and fry my device?  If it is automatic, is there a reason to not just have all ports 48v enabled and have them use it as needed?

Thanks!