The gasket is made from Silicone and is required for the connector to meet IP67 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:
USB4715 - Waterproof IP67 USB Type C 6 Contact Connector
USB4730 - Waterproof IP67 USB Type C 16 Contact Connector