Dear Corey_1,
After receiving the GATE message, the ONU stores the parameters such as startTime, and sends the data according to the stored parameters when the local MPCP clock reaches the time point. The ONU has a buffer. The pending grants indicate the remaining space in the buffer and are used to tell the OLT how many more grant windows can be buffered, if the number of packets exceeds the threshold, the ONU discards the excess packets.

I hope it helps. Thank you!