A race track is split up in sectors. Each sector is taken care of by a group of marshals. They keep it clean, give flag signals, help out when a crash happens, call in dangerous driving, ...

A sector starts at a marshalpost and ends at the next marshalpost. These posts can always see eachother. This way, every inch of the track is seen by marshals and the posts can react to the previous or next posts flag signals.

When two posts are positioned on the other side of the track, often an arrangment is made so that one post takes of one side and the other post of the other side of the track. Doing this will make crossing the track unneccesary. Of course, this is only possible when enough people are available because the length of your sector at one side will be expanded.

Marshals and pilots split up the track in a different way. A pilot will give a number to each corner, a marshal will give a post a number.

Here you can see some track layouts and the marshalposts.

Circuit Zolder

As of 2015 the numbers of the marshals posts are changed on request of the FIA. We used to number these posts in the order they had next to the track. The new method of numbering follows the numbering of the turns. Just like pilots do when they communicate with their team.
When there are posts on a straight line, they get the number of the corner at the beginning of the straight line, followed by a letter (A, B, ...). You'll  notice the 4th corner has post 4, followed by posts 4A and 4B, both placed on the straight. The next corner is the chicane and there we now have post 5 (because it is turn 5).
The attentive reader already noticed there is no post number 9 and 13. Because turn 9 and turn 13 do not have a post for marshals.

This is the new numbering of marshals posts:
Circuit Zolder (Belgium)

This was the numbering before 2015. Pay attention to the orientation of the track. This image is turned by 180°.
 Circuit Zolder (Belgium)

Circuit Spa-Francorchamps

 Spa-Francorchamps (Belgium)

Brands Hatch

Brands Hatch (United Kingdom)