Sealing plugs are used to seal the connector when all the cavities are not used by wires. It is not required to seal open cavities from a function standpoint, however if you want it to be waterproof and provide a layer of protection beyond what the boot provides it is required.
THERE ARE 2 SIZES:
RED/SMALL - SOLD IN PACKS OF 10
WHITE/LARGE - SOLD IN PACKS OF 4
HERE IS WHAT YOU NEED FOR YOUR CONNECTOR:
47 PIN- ALL CAVITIES CAN BE PLUGGED BY RED/SMALL
29 PIN- 20/16 GA CAN BE PLUGGED BY RED/SMALL
THE (4) LARGE 12 GA CAVITIES REQUIRE WHITE/LARGE
35 PIN- 20 GA CAN BE PLUGGED BY RED/SMALL
16 GA CAN BE PLUGGED BY WHITE/LARGE
16 PIN- ALL CAVITIES REQUIRE WHITE/LARGE
18 PIN DTV- CAN BE PLUGGED BY RED/SMALL OR WHITE/LARGE