It depends. If you stick the custom rom file in a system folder by itself, it will just not work. The file I made stores the required data in the bootfiles folder. It would not even begin to boot up normally. If you somehow boot up a system folder that has the custom rom file and bootfiles folder with another rom file (maybe by having that rom file on a partition without a system file and finder), it will behave just as a normal system folder. There is nothing in the normal startup code that looks in the bootfiles folder, so it is just ignored. The rom file would normally be consulted for a few things, but it has no resource fork so those consultations fail. Mac OS 9 seems to be ok with that. You could have a system that absolutely has to have some of the resources (or datafork pefs) stored in a regular rom file, but I have not come across one of those systems yet and have no idea which ones they would be, if any. I hope that this answered your question.