A Feature Studio is a tab containing FeatureScript, a programming language used to define your own custom features in Onshape.
FeatureScript is designed by Onshape for writing features, and more generally, working with 3D parametric models. The language is built into Onshape from the ground up, providing the foundation of Part Studio modeling and used to define Onshape standard features (like Extrude, Fillet and Shell).
For detailed information on how to use FeatureScript to create custom features, see Welcome to FeatureScript.
For detailed information on using custom features within your Onshape account, see Custom Feature.
See Sketch tools, Feature tools, and Assemblies for more information on customizing those toolbars.
For an additional Learning center resource, follow the instructor-led training course here: FeatureScript Fundamentals (Onshape account required).