InterForm400 is able to print out on both laser printers and label printers connected to the IBM i.
InterForm400 can generate the print output in many different printer languages - selected as a printer type on the merge.
You can connect the printer with the IBM i in as good as any way you want, but the manual does include some recommendations. The result of a merge is a new *USERASCII spooled file, so InterForm400 is not involved in the actual printing process.
The print output can be a changed, unformatted EBCDIC spooled file, but usually you want to merge an input spooled file with an overlay or overlay selector.
The contents of the merged spooled files can normally not be displayed, but it is possible to view them with Swiftview.