In general, most double tires equipped in truck vehicles have a hub seal (inner) and a shaft seal (outer).
The hub seal prevents grease from leaking into the hub bearing and also prevents water and foreign substances from penetrating from the outside. They require press-fit when installing it.
The shaft seal prevents leaks of hub bearing grease and also prevents differential oil from penetrating into the hub bearing. It does not necessarily require press-fit. It is strongly recommended that the hub seal (inner) and shaft seal (outer) be replaced at the same time.