Addendum (not relevant for VATSIM)
In addition to Approval Request, Release and Reference, there are other types of coordination. However, for various reasons, these are neglected on VATSIM. Nevertheless, they will be presented briefly.
During an estimate call, squawk, handover level and (entry) time are exchanged. Nowadays, most estimates are automatically exchanged between sectors via flight data systems. However, in the event of a system failure and in some other cases, it is still necessary to verbally phone through this Estimate. As an example, let's take a flight from Frankfurt to Munich and assume that the automatic system is not available and therefore all estimates have to be coordinated verbally: As soon as the flight takes off, the tower calls Departure and reports the departure time. The departure controller and all other controllers along the planned flight path already have flight plan data and a corresponding flight strip (but without times, as the exact departure cannot be predicted). The departure controller then calculates the time of the flight at the COP between departure and centre and passes this on in an estimate conversation:
ESTIMATE CINDY DLH123
||A320 to EDDM
||SQUAWKING 1000, ESTIMATED CINDY 1023 CLIMBING FL130 <initials>
The matching of aircraft type and destination is used to ensure that you are talking about the same flight and have the same flight plan. Now the centre controller can calculate the time and level at the COP to the next centre and pass this through accordingly.
On VATSIM ESTIMATES are obviously not necessary, as the times are hardly relevant and the data exchange via Euroscope is always ensured.
Estimate - no details
An "Estimate - no details" is a modified form of an estimate where the receiving sector has no flight plan data. This is necessary, for example, in bad weather when a flight has to enter a sector that is not originally on its route. In addition to squawk, time and level, aircraft type, speed, requested level, departure, destination and route are exchanged.
An expedite clearance is a "short-term estimate" and has the character of an approval request. If the flight time to the sector boundary is less than a value specified in the agreements, an Expedite Clearance must be coordinated instead of an Estimate.
A revision is sent if there is an early change in time, level or other parameters.