Singhal–Kshemkalyani’s differential technique is based on the observation that between successive message sends to the same process, only a few entries of the vector clock at the sender process are likely to change. This is more likely when the number of processes is large because only a few of them will interact frequently by passing messages. In this technique, when a process Pi sends a message to a process Pj , it piggybacks only those entries of its vector clock that differ since the last message sent to Pj.
I am understanding above but not the following statement
this technique requires that the communication channels follow FIFO discipline for message delivery. link
Chapter 3 -Logical clock from above book and sub topic is Efficient implementations of vector clocks