Handy for 3 basic purposes:

1.  Marking film prior to installation of windows with unusual shapes (arches, trapezoids, etc.) which must be cut to approximate shape before the final "lay-up."
2. Circling a contaminant (hair or fiber) before pulling back the film to remove it.
3. Marking film edges to check for glass movement on side roll-ups during the trimming process. Yellow grease mark easily wipes away.