I’m starting a project to develop an extension for SketchUp and would like to know if I use the Sketchup SDK or Ruby for development?
The SDK is for reading and writing .skp files in external applications.
The Ruby API is for extensions that are loaded into SketchUp. The Ruby language itself can be extended with C/C++ code (“Ruby C extensions”), but that needs to be loaded by a Ruby script.
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“Creating Extensions for SketchUp uses the Ruby API. Whether you’re creating plugins, extensions or add-ons (synonyms for the same thing) then you’ll need to know some Ruby.”
“If you’re working creating importers or exporters for SketchUp then you’ll want to work with the SketchUp SDK.”
However “SketchUp for Schools” (what is in your profile ) is a browser-based SketchUp and does not able to run any extension currently.
I think you have to be more specific to be able to get proper answer from the experts. (Not like me… 
)
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