Barge boards fulfil different purposes and team employ different strategies to gain aero efficiency and even downforce.

Initially Barge boards were introduced by McLaren, Long sidepods were favoured at that time in order to smooth the airflow to the rear wing and Ferraris 640 had sidepods extending towards the front of the flat bottom, the downside of this was as ground clearance was decreased the longer flat bottom choked as the boundary layer built up. McLaren aimed to shorten the underbody flat bottom length while still flowing air efficiently around the car. Barge boards were effectively long sidepods with the tops and bottoms cut off.
Most teams followed suit and the familiar format bargeboard was created. Barge boards were simple vertical boards shaped to flow the air. They helped the direct flow off the outer ends of the front wing away from the central bodywork of the car and create a calmer airflow for the rear wing, it also produced a cleaner airflow into the sidepods for improved cooling there was a small benefit in that the sidepods inlets were protected from debris thrown from the front wheels.

Then in the past few years both Ferrari and McLaren have sought to gain other benefits from the bargeboards, they have been brought closer and moulded to match the sculpted side of the chassis, the aim being to create a divergent duct with the air gap, this produces a lower pressure under the raised nose of the car. Ferrari did this by heavily shaping the barge boards and McLaren by shaping the underside of the raised nose. Now most teams have complex barge boards with heavily moulded shapes and extra extractors in line with the apex above the shadow plate\splitter to pull air from above the splitter.
The teams with the more complex barge boards (e.g. McLaren & Prost) extend the front of the boards ahead of the flat bottom this then means a shadow plate isn’t required along the bottom of the board. The teams also curve the barge board under the raised nose and hence close enough to the centre line of the car to be inside the step plane area and hence lower the bottom edges of the boards down lower to the reference plane.
he teams also incorporate the vertical fences under the front wing to direct the airflow from under the wing to interact with the bargeboards.

Williams and Jordan have different strategies aimed more at maximising the flow off the front wing and with no barge boards directly ahead of the sidepods, Williams have used low (10cm) curved deflectors mounted around the front of the front shadow plate\splitter to direct airflow around and into the sidepods. Interestingly (or at least coincidentally) to note both these cars use the horizontal fins low down ahead the sidepods inlets, these fins act to both prevent too much air passing under the car and also to improve the quality of the airflow that does make it under the car.

The FIA dimensional regs’ regard barge boards and most other aero devices simply as bodywork. The regulations set out in plan and side view the areas where bodywork can be placed, they also set out the heights and widths for the step plane (bottoms of the sidepods) above the reference plane (bottom of the middle of the chassis). Bargeboards can only be placed so far from the centre line of the car

3.7 Front bodywork height :
All bodywork situated forward of a point lying 330mm behind the front wheel centre line, and more than 250mm from the centre line of the car, must be no less than 100mm and no more than 300mm above the reference plane.

3.11 Bodywork around the front wheels :
With the exception of brake cooling ducts, in plan view, there must be no bodywork in the area formed by two longitudinal lines parallel to and 400mm and 900mm from the car centre line and two transversal lines, one 350mm forward of and one 800mm behind the front wheel centre line.