If a switch receives untagged Ethernet frames on its Trunk port, they are forwarded to the VLAN that is configured on the Switch as native VLAN. Both sides of the trunk link must be configured to be in same native VLAN.
Consider the below example. The trunk link connecting between the Switches, omnisecu.com.SW1 and omnisecu.com.SW2 are connected using a Hub. Some computers are also connected to the Hub. Hubs and computers are not aware of tagging protocols and the Ethernet frames originating from Hubs are untagged Ethernet frames.