In brief;

• The advertised 'start times' are race 'start times'. In readiness for a race start, riders need to be on the grid ready to go.
• This means Call-ups will commence approx 10 minutes prior to the advertised start time. Riders need to be in position ready to be called by the call-up commissaires.
• To keep the day on track & fit the racing in, there needs to be an amount of overlap between the preceding race finishing and rider call-ups for the next race when races are back-to-back.
• Riders will assemble in the call-up marshaling area awaiting direction from the call-up Commissaires
• Riders will be called forward by the Commissaire in order, and take their position on the startline.
• When called up, Riders will be advised what row they are on. They get to select their preferred start slot on their row (from what is still available).  
• Riders cannot move up into spare slots unless advised to do so a Commissaire. Please understand that this is not an automatic right and the startline may not allow time for an explanation regarding the background to the commissaire's decision.
• Riders have a responsibility to be on time & to listen for their call-up.
• Not being ready for call-ups may result in forfeiture of your grid position (depending on extent of lateness).
• C2 riders will have their tyre widths checked at call-up for compliance to 33mm standards using a UCI measuring gauge.
• Rider call-ups for Sunday will be re-seeded overnight Saturday to take account of Saturday's race results.
  

Source: UCI Cyclocross Regulations
(See http://www.uci.ch/mm/Document/News/Rulesandregulation/16/61/43/5CRO-E-03.06.2015_English.pdf Pg 20)