What format do custom coordinate system files need to be in in a shared directory

From Fmepedia

Q) What format do custom coordinate system files need to be in in a shared directory?

A) When used as a Shared Resource coordinate systems should be stored in a plain text file with the same structure as MyCoordSysDefs.fme and LocalCoordSysDefs.fme. The file extension type can be anything - any file that is placed in a shared custom coordinate system folder will automatically be scanned for valid coordinate systems.