Workstation 15 won't allow me to import certain .ova files ... ?

I'm a VMware newb. Let's get that out of the way right up front. But I did honestly try to search through previous topics before starting this one at least.

In a small lab environment, I'm running Workstation 15 on a Windows 10 host.  I do not yet understand

1. the difference between .ova and .ovf files
2. why it is that some .ova files (example:  EMC's Isilon Simulator) import well into Workstation, but others (i.e. EMC Data Domain Virtual Edition) do not ?

I did succeed in building up a small vSphere environment within Workstation using a nested ESXi host, but I'd kinda, sorta rather keep all of the VMs at the same level (i.e. within Workstation.)

Help?   thanks!