combination waste and vent ?

Here in NC, we use the IPC with NC amendments. According to our code, in a combination waste and vent system,

"A vent shall be provided at the upstream end of each branch, washed over or under by the last fixture on the branch. ..."

What exactly do they mean by washing over or under?

I believe they are saying that it needs to be either directly washed by a fixture (sink with vent) or that the vent has to be downstream of a fixture so the base is washed. No dry vent (or at least what used to be called a dry vent) i.e. dead end.

What they're saying is that you need to have a fixture upstream of the "combination vent" (wet vent to me) to keep the vent clean from build up.

They chose sink, etc, because they're frequently used fixtures.