The various models of Spectrum supported a range of ways to connect printers, three of which are supported by Fuse. Different printers are made available for the different models:
ZX Printer
Serial printer (text-only)
Parallel printer (text-only)
If +D emulation is in use and printer emulation is enabled, text-only emulation of the +D's parallel printer interface is provided.
Any printout is appended to one (or both) of two files, depending on the printer - these default to printout.txt for text output, and printout.pbm for graphics (PBM images are supported by most image viewers and converters). These names can be changed with the Options, Printer... menu option. While the ZX Printer can only output graphically, simulated text output is generated at the same time using a crude sort of OCR based on the current character set (a bit like using SCREEN$). There is currently no support for graphics when using the serial/parallel output, though any escape codes used will be `printed' faithfully. (!)
By the way, it's not a good idea to modify the printout.pbm file outside of Fuse if you want to continue appending to it. The header needs to have a certain layout for Fuse to be able to continue appending to it correctly, and the file will be overwritten if it can't be appended to.