This is a kind of a collection of objects where the system can walk through. E.g. when multiple handles are selected and you want to apply a certain function to each of them, IEnumerable is probably the type this functions needs to cope with.

Example Patch - Freeze selected fixtures:

      <step>Programmer.Editor.Selection.GetSelectedHandles("Windows.PatchView.Handles")</step>
      <step>Programmer.Editor.Fixtures.Patch.FreezeFixtures(Windows.PatchView.Handles, True)</step>

The first step puts the currently selected fixtures into a specific property (in that case, “Windows.PatchView.Handles”).
The second step applies a function - which is capable of handling an IEnumerable - to this property. In this case, it sets all fixtures which are in this property to 'frozen'.