When implementing an Inter-Switch Link (ISL) between two HPE SN6000B switches, the correct port mode that must be set to enable communication between the switches is E_PORT. E_PORT stands for Expansion Port and is used to connect switches in a fabric. It facilitates the creation of an ISL that allows traffic to be routed between the switches in the fabric, enabling the switches to share the connected devices' routing tables and maintain a unified fabric.