Lexmark 4I3 - Manual (Page 171)

The Document Insertion Device (F3) has an additional guide called a
lower forms guide that fits over the sensor.
L o w e r
F o r m s G u id e
The lower forms guide works as:
A guide for aligning the cut sheet form.
The sensor works as:
A sensor for detecting the presence of a form and activating the
Load/Eject key.
Chapter 3. Getting to Know Your Printer 3-7