The original implementation of PostScript fax on the LaserWriter Select 360 and LaserWriter Pro 810 printers did not have the "intelligence" to check/select the appropriate paper tray based on the incoming fax.
There is a "LaserWriter Fax Tune-up" that resolves a two-page fax problem where two pages are printed when only one page was faxed. This tune-up also adds the intelligence outlined above to route faxes to the appropriate tray based on the page size of the incoming fax. In other words, if a Legal-sized fax job comes in and there is a Legal paper tray installed, it will print to the Legal tray.
Note: The fax tuneup is downloaded into RAM so if the printer is power-cycled, the tuneup MUST be launched again to take effect.
Note: This article mentions an Apple software update available from the Apple Software Updates Web site at
http://www.apple.com/swupdates.