[MRCA] PCS Mode

[Alex J Rokowetz]

The Transworld RT7000 HF transceiver has the following modes:  USB, LSB, USB Data, LSB Data, AME, CW and one mode with which I am completely unfamiliar... PCS.  The manual states that PCS is the "same as USB but with the addition of a carrier signal in transmit mode at a level of -16 dB relative to PEP".  The manual provides no other information about the PCS mode or when it would be used.
 
I'm looking for additional information about this mode and searching the internet has so far yielded no results.  Does any one have any familiarity with PCS or in what situations it's typically used?
 
Thank you,

Alex
K2AJR