What is FBtoC?

FBtoC is a replacement for the FB Compiler (FB Compiler.app). Translating the code to C allows Apple's developer tools to create a universal binary. The diagrams below illustrate the process:

FB Compiler 

FB source code ------> legacy app (PPC-only, CFM format, resource fork...)

FBtoC 

FB source code ------> C source code ------> OS X app package with Carbon universal binary in Mach-O format
FBtoC translates appearance projects and standalone files, and console-type text-only standalone files. PG projects cannot be translated.

Using FBtoC normally requires no detailed knowledge of C.