Reference

Anything that cannot be handled with an approval request or a release falls under the coordination type "Reference".

The most popular application for this is a request to the upstream sector. Whenever I want a flight to fly into my sector that deviates from the standard, a reference call is made.

EDMM	REFERENCE DKB DLH123 REFERENCE 20 MILES WEST OF DKB DLH123
EDGG	Go ahead
EDMM	REQUEST HIM DIRECT LANDU REQUEST HIM DCT DM424, DESCENDING FL150 REQUEST HIM AT FL210 REQUEST HIM AT SPEED 250 KNOTS
EDGG	CONSIDER <initials> WILCO<initials> UNABLE <initials>
EDMM	<initials>

The responses CONSIDER and WILCO have the same meaning: The request is fulfilled by the upstream. Since the Recieving Unit decides on the conditions for entry, a request should only be rejected or renegotiated in exceptional cases. Here, too, common sense should be used to find a solution that suits both partners.
A few words about the "CONSIDER", which at first glance seems somewhat confusing. In German, people like to say "CONSIDER" instead of WILCO. This does not mean to think about it, but stands for "CONSIDER IT DONE".

In some situations, it also makes sense as the sending sector to make a request with the help of a reference call. You then make an open request instead of an approval request:

Reference <COP/position> <callsign>
Request higher/lower level

Typically, this is used for a crossover from APP to CTR or from Lower CTR to Upper CTR (or vice versa). Such calls are useful when, due to traffic, the departure/arrivalmust arrive at its actual exit level before a handoff can take place because a crossing over/under has not been made until the appropriate level has been reached. Over a higher/lower level, continuous climb and/or one-stop problem solving is possible.