Servicing occurs when a vehicle enters a Depots or Depots. This prolongs the reliability rating (reducing breakdowns) and increases the working life of the vehicle.
Vehicles should automatically head to a depot or hangar when they reach the end of their service interval. This can be changed in the vehicle details, or in the patch options.
You can also make vehicles go to a depot to service routinely by adding the depot to the vehicle's orders.
This way you service the vehicle more regularly, and can help prevent Breakdowns. It also avoids the possibility that a vehicle may need servicing at an inconvenient point on its route (maybe the nearest depot is a long way away).
This type of servicing should be scheduled before a train loads its cargo to ensure the cargo isn't delayed in reaching its destination (reducing the payment).
If you do not want the vehicle to visit the depot every time the orders list is executed, you can use the Maintain resp. Service instruction on the goto depot order - this causes the order to be ignored if the servicing interval has not yet expired.
Always go Option
The vehicle enters the depot and get serviced everytime it finds the depot order in its order list.
Maintain if needed
The vehicle enters the depot when it has reached the service interval and the depot order is the next on the order list.
A vehicle with a goto depot order anywhere in its orders will never look for a depot on its own, regardless of the service interval set. So if you set one goto depot order, you become responsible for all of that vehicle's servicing.
Forced servicing (trains only)
Trains can be forced to enter a depot by laying tracks in a way that gives them no choice but to enter a depot. Since there is a noticeable decrease in speed while a train enters and leaves a depot, the number of servicing-lanes should be appropriate to the amount of traffic.
Forced servicing depots can be placed in pairs on either side of the track, so that while one depot is blocked by an exiting train, the next train can already enter the other one. It is important to place a path-based signal before the entrance to the depots — otherwise the incoming train will have to wait until the whole signal block is free, including depot exits.