What's the difference between "ThisDocument" and "ActiveDocument"?

Here's a sample macro. Where would I put your code line?

Sub RevSummary()
'
' RevSummary Macro
'
'
Application.Templates( _
"C:\Users\robert.ross\Documents\Work\Nawah\Mas ter Templates\New\Nawah Procedure.dotm" _
).BuildingBlockEntries("Revision Summary").Insert Where:=Selection.Range, _
RichText:=True
End Sub