A test scenario is a test case for a use case / story in a business domain. There may be one or many test scenarios for every acceptance criterion defined for the story.
Action Steps :
An action step is generally an atomic transaction as perceived by users and business experts.
Step Units :
A step unit is a technical decomposition of an Action Step. These details are generally abstracted from an end user or business domain experts seeing that in its entirety. These are not allowed to be provided for manual actions steps.
Here is the link to help documentation.