This is the link you may be looking for:
http://www.innovationfirst.com/FIRST...ence_Guide.pdf
However, this only says that you should not use 255 as a PWM output. Jnadke is correct two bytes of 255 will signal a new packet to the output processor in the RC. This is also why both relay outputs are spaced out by the PWM outputs (relays can be 255). Your code has worked with a 255 pwm output because there were never two in order.