Printer types is a list of the types of printers into which the print filter can convert files. For most printers and filters, you can leave this part of the filter definition blank, because it is identical to the list of output types. But it can be different. For example, you could have a printer with a single printer type for purposes of initialization, but which can recognize several different file content types. Essentially, this printer has an internal filter that converts the various file types into one that it can handle. Thus, a filter might produce one of several output types that match the file types that the printer can handle. The print filter should be marked as working with that printer type.
As another example, you might have two different models of printers that are listed as accepting the same file types. Due to slight differences in manufacture, however, one printer deviates in the results it produces. You label the printers as being of different printer types, say A and B, where B is the one that deviates. You create a filter that adjusts files to account for the deviation produced by printers of type B. Because this filter is needed only for those printer types, you would list it as working only on type B printers.