The keyword far is a synonym for __far, which is not allowed when the -Ansi: Strict ANSI compiler option is present.
A __far pointer allows access to the whole memory range supported by the processor, not just to the default data page. You can use it to access memory mapped I/O registers that are not on the data page. You can also use it to allocate constant strings in a ROM not on the data page.
The __far keyword defines the calling convention for a function. Some backends support special calling conventions which also set a page register when a function is called. This enables you to use more code than the address space can usually accommodate. The special allocation of such functions is not done automatically.