Definition of Terms

Runtime

Runtime is then, when scenarios are executed by a test runner

GeneratorTime

GeneratorTime is then, when the code-behind files are generated.

Code-Behind files

For every feature file, SpecFlow generates a code-behind file, which contains the code for the various test frameworks.
It’s generated when the project gets compiled. This is done by the SpecFlow.Tools.MsBuild.Generation MSBuilt task.