The Scenarios page in the Project Settings dialog, allows you to choose one of the following two options.
When this option is selected, a Slide file will consist of a single model, with filename extension *.slim (compressed) or *.sli (uncompressed).
This is equivalent to the method used in Slide version 6.0 and earlier versions.
When this option is selected, a Slide file may consist of multiple models (groups and scenarios), within a single file. The filename extension will be *.slmd (slide multi doc file, compressed).
Multiple groups and scenarios allow you to simultaneously create, edit and analyze multiple variations of a Slide model, with different geometry (for example) and/or different input parameters (e.g. material properties, support, loading, groundwater). All groups and scenarios are saved within a single *.slmd file, which can be edited, computed and interpreted as a single file.
In Slide 2018 this is the default modeling option.
For more information see the Multi Scenario Modeling topic.