The gasket is made from Silicone and is required for the connector to meet IP68 standards. Ordered separately or as a KIT, it must be fitted after the connector has been soldered to the PCB.
This gasket fits the following USB connectors:
USB4735 - Waterproof IP68 USB Type C 6 Contact Connector USB4740 - Waterproof IP67 USB Type C 6 Contact Connector