FusionSDK
The FusionSDK is a BMD's software development kit to create Fusion Plugins using C/C++ code.
The FusionSDK must be licensed under an NDA!
The FusionSDK must be license from BMD under an NDA (Non Disclosure Agreement). The FusionSDK costs you $0 but has a signed PDF contract where you agree not to share BMDs header files from the SDK with others.
The .plugin format allows low level access to Fusion's 3D workspace. You can also make plugins that have similar features to what is possible with an OpenFX plugin for 2D graphics.
A Fuse and an OFX (OpenFX) addon are a great way to get started with making custom node based tools in Resolve/Fusion.
Plugins do work only in the Studio editions.
The .plugin files only work in the payed "DaVinci Resolve Studio" and "Fusion Studio", not in "DaVinci Resolve" (free).
The Krokodove plugin in Reactor is an example of a compiled FusionSDK based Tool, and so is LearnNowFX's AccuShaders PBR surface material system which is available on Gumroad. Both of those tools make excellent use of the Fusion page's 3D workspace.
Being compiled software OpenFX plugins and FusionSDK plugins makes it easier to put a paid license management feature in place if releasing paid commercial software.