As you've already surmised,

df.zoomToSelectedFeatures()

will change the extents of the data frame to all selected features in the map frame. If you're interested in just zooming to a selection set for a specific layer then use "lyr.getSelectedExtent()". I also adjust the map scale factor so my code either looks like this:

df.extent = lyr.getSelectedExtent()
df.scale *= 1.5
arcpy.RefreshActiveView()

or this:

df.extent = lyr.getSelectedExtent()
df.scale = 12000 # 1:1,000
arcpy.RefreshActiveView()

As you've already surmised,

df.zoomToSelectedFeatures()

will change the extents of the data frame to all selected features in the map frame. If you're interested in just zooming to a selection set for a specific layer then use lyr.getSelectedExtent(). I also adjust the map scale factor so my code either looks like this:

df.extent = lyr.getSelectedExtent()
df.scale *= 1.5
arcpy.RefreshActiveView()

or this:

df.extent = lyr.getSelectedExtent()
df.scale = 12000 # 1:1,000
arcpy.RefreshActiveView()