FG140D: Difference between VLAN Switch, Software Switch and Virtual Switch?

[NapaCab]

Hi,

I'm just getting my feet wet on a Fortigate 140D-POE running FOS v5.2.5, and when I go to configure interfaces, I see a few options available for switching.  One is VLAN Switch (which is what my switchports were defaulted into on the Fortigate) and the other is the Software Switch.  I also see in the FortiOS guide that there's an option for a Virtual Switch.   Could I please get some clarification as to the differences between these three switch entities?

Also I noticed on my FG140D that the switchports default into VLAN Switch (1) (VLAN ID: 0).  If I try to change this VLAN ID 0 to say VLAN1, I get the following puzzling error message:  "Please enter a value greater than or equal to 100."

What's going on here?

Thanks, and please be gentle, this is my first post :)