The gasket is made from Silicone and is required for the connector to meet IP67 standards. Ordered separately, it must be fitted after the connector has been soldered to the PCB.
This gasket fits the following USB connectors:
USB3500 - Waterproof IP67 Mid Mount Micro USB 2.0 Connector
USB3505 - Waterproof IP67 Top Mount Micro USB 2.0 Connector
USB3510 - Waterproof IP67 Vertical Mount Micro USB 2.0 Connector