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 Horizontal Micro USB 2.0 Connector USB3510 - Waterproof IP67 Top Mount Vertical Micro USB 2.0 Connector