[1] It pre-dates the Bell–LaPadula security model, whose first volume appeared in 1972.
Under high-water mark, any object less than the user's security level can be opened, but the object is relabeled to reflect the highest security level currently open, hence the name.
The practical effect of the high-water mark was a gradual movement of all objects towards the highest security level in the system.
Finally, if user C is assigned to assemble the daily intelligence briefing at the TOP SECRET level, reference to the dictionary makes the dictionary TOP SECRET, too.
In the Biba model, no-write-up and no-read-down rules are enforced.