I have just received a "Digi XBee SX RF Module Dev Kit" and reading the User Guide it says "Connect the SX development boards to the USB port on a PC via the mini-USB cables. Separate the SX development boards by at least 2 m (6 ft).". What happen if two XBee SX are communicating each other by a distance less than 2 m? Is this warning only for XBee-PRO SX modules or for XBee SX too? Thanks.