Do you mean a Race Course?
- if you're discussing a race course, the numbers would be designating that course: marks are previously identified with numbers by the race committee. Then, when wind/conditions dictate a certain race course for the event that sequence of identified marks is communicated to the competitors. (thus, in your example, the fleet would race from the start line to mark "3", then mark "4", then mark "2" -then to the finish. Typically, race instructions will include general instructions, such as "all marks to be rounded to port unless otherwise specified").
Maybe I've overstating the obvious, but it's hard to know just what puzzle we're trying to solve here without more info!
Regards,
JPC