ultrasonic n RF

1. what's the difference between ultrasonic and RF?

2. 27 Mhz is VHF or HF?

3. if 1 tx/rx provides 4 commands, can i use 2 tx/rx board (27 Mhz n 33 MHz) in order to get 8 commands using the same antenna?

Ultrasonics are sound waves having frequency greater than 20kHz whereas RF are electro-magnetic waves. They both differ from each other by their properties. Sound waves travel by compression and rarefactions of air or any ohter medium whereas EM waves progogate in the same way as light does. They does not require any medium for propogation.