The DayStar 100mm Energy Rejection Filter is for on-axis use with a refractor and/or Maksutov-style telescope OTA. This filter has an outer diameter of 120mm, fitting around your telescope OTA, while the filter window itself has a diameter of 100mm.
Energy Rejection Filters reduce the heat load on your filter assembly by absorbing or reflecting UV and/or I light and transmitting light in the desired visual spectrum. These filters are sometimes Red or Yellow glass, or dielectric IR and UV, but pass light in the desired visual spectrum. The ERF can also serve as an aperture mask to accomplish a straight angle of incidence for light entering the filter.
DayStar filter assemblies operate best at f/30 and slower.