What's the difference between a 'standalone' and an 'autonomous' edge?
Both are referenced as different deployment scenarios in this NSX-T 3.0 Admin guide:
Standalone edges are deployed only in a vSphere Environment, autonomous can be deployed in on prem or public cloud.
I also asked VMware support this question, as I had a ticket open anyway. They explained that the Standalone Edge is a component of NSX-V and the Automous Edge is a component of NSX-T. Both require a vSphere Hypervisor.
The NSX Standalone Edge is available for download for both NSX Data Center for vSphere and NSX-T Data Center products.
The NSX Standalone Edge is compatible with previous releases for customers who have workflows using this solution and do not want to adopt the newer high performance autonomous Edge.