To add to the confusion, passenger missions within the bubble adds less to the ranks than long range passenger missions outside the bubble.
There are 2 kinds of passenger missions: "regular" and "sightseeing". The later always have a client name in the mission title, and the recommended rank in the description is an Explorer rank. The pasengers for such a mission also occupy an entire cabin no matter the party size. The "regular" ones do not have any specific name (just passenger type), recommemded rank is based on Trade rank, and fill up cabins as normal.
Sightseeing passenger missions count towards Explorer rank but regular passenger missions do not.