1. When a DataSourceSelectionID is needed?
No ID is needed (green circle) when there is only one column in the data source of the drop down (blue circles)
The selection ID is a name for a specific row in a data source that is being singled out based on user selections (yellow)
ID is needed below because there is more columns in the data source
The ID used by the system as a reference to a specific row in a Data source

2. When to use SelectionID in a tag string in a plugin
Userprofile.FieldName.ColumnName
VS.
UserProfile.SelectionID.ColumnName