Pit board jokes aside, here is a good article:
F1 teams told coded instructions will not be allowed either - F1 news - AUTOSPORT.com
EXAMPLES OF WHAT IS AND WHAT IS NOT ALLOWED
Allowed
- Telling a driver when he is allowed to or meant to stop for new tyres
- Team orders in terms of overtaking or letting a team-mate past
- Warning a driver about the traffic situation during qualifying and the race
- Safety warning relating to yellow flags or dangerous situations on the track
Not allowed
- Informing a driver to change any settings on his car to improve its performance
- Informing a driver about his fuel consumption and how to manage his pace
- Informing a driver about his own performance in sectors and where he can improve
- Informing a driver about the condition or state of his tyres
- Informing a driver how best to approach the formation lap or setting changes to make better start