Classes define the applicability, or scope, of a rule instance. A data class is typically used to describe the data model, and can be used for interfacing with the data base. It pattern inherits from Data- and will contain properties common to this class.So if there is a need to refer these properties in the section you might think of creating a section directly there .
If you have repeat grid, containing list of "data" items. The repeat grid can be configured to use "Expand" pane, which can provide more details on the row. The expand pane flow action / section would need to come from the data class.
If you have a repeat grid an use a "refresh section" action and use a data transform or activity on that refresh, the data transform and activity need to be in the section's class (so it's often easier to have the section in your Data- class if your DT's and activities are already there).