why is there an empty token in the 4-phase handshake protocol of asynchronous circuit

[sevid]

hi, everyone

in the field of asynchronous circuit, it says that there is an empty token in the 4-phase handshake protocol, but not in the 2-phase handshake protocol, i really want to know why. just because the level-triggered return-to-zero request and acknowledge handshake signals?

thanks in advance.

jeffrey