It's the FCode-Number limit, ie. 0x0fff. For details please look at: OpenFirmware IEEE-1275 (1994) spec. section 2.3.43.
If I got this right, I am allowed to define/declare only 0x0fff number of constants and/or functions, variables, defers etc. for which the tokenizer assigns a FCode-Number in the final tokenized FCode image.
Some Open Firmware implementations have a special FCode x'f4 which allows FCode#s up to x'ffff; x'f4 followed by "high" byte, "low" byte.
Segher