6" above the highest fixture itself

It is to make sure the vent stays dry and does not have waste in the line.

A vent line may not run horizontal until it reaches an elevation at least 6" above the highest fixture served by that vent.

The same rule applies to a stack. You may not attach a vent line to a stack until you reach an elevation at least 6" above the flood level rim of the highest fixture served by that stack.

By example, if you are venting a bathroom on the first floor and you need an auxiliary vent, the horizontal offset of that auxiliary vent would need to be at least 6" higher than the lavatory bowl rim in that bathroom. If you want t6o attach that vent to the stack serving both the first and second floor you cannot attach the auxiliary vent to the stack until it reaches a point on the second floor which is 6" higher than the flood level rim of the highest fixture served by that stack on the second floor.

Also, Under the UPC vents are sized by the dfu's load of the fixtures served by that vent, but if there is a horizontal offset longer than 3" the entire length of the vent must be increased one nominal trade size.