Output types is list of file types that the filter can produce as output. For each input type, the filter produces a single output type. The output type can vary, however, from job to job. The name of the output type is restricted to 14 alphanumeric characters and dashes.
The output type names should either match the types of available (local or remote) printers, or match the input types handled by other filters. The LP print service groups filters in a shell pipeline if it finds that several passes by different filters are needed to convert a file. You will unlikely need this level of sophistication, but the LP print service allows it. Try to find a set of filters that takes as input types all the different files the users might want printed, and that converts those files directly into file types the printer can handle.