Gaussian 03 Online Manual
This keyword allows you to specify options for Periodic Boundary Conditions jobs. Note PBC is turned on simply by including translation vectors in the input structure, and this keyword is used only to control how PBC calculations are performed. If you do not need any of these options, you do not have to include the keyword to perform a PBC calculation.
See the "Specifying Periodic Systems" subsection of the "Overview of Molecule Specifications" section.