Coordination is indispensable in the overall air traffic control system. This is particularly important in a highly complex airspace such as that over the Federal Republic of Germany with a very large number of small sectors. Many problems cannot be solved in one's own sector and must accordingly be regulated in cooperation with adjacent sectors. The basis for coordination between the different sectors is provided by the various operating agreements and letters of agreement (LoA). For many of the standard flight movements, conditions for the handover of aircraft at sector boundaries can be found there. However, for a variety of reasons, it is often necessary to deviate from the standard agreed in the documents. In this case, coordination with neighboring sectors is required. In real air traffic control, almost every center and approach working position has a so-called coordination controller or planner who, among other things, has the task of telephoning neighboring sectors and thus carrying out coordination.
In air traffic control, the principle of "the receiving unit states the conditions" applies, i.e. the receiving sector determines the conditions under which aircraft must enter the sector. In extreme cases, this also means that in a fictitious sector sequence "A-B", controller B says that all aircraft should now fly in "at FL100, Speed 220, DCT <WPT>", then sector A must ensure this.
This does not mean that individual or general renegotiation is not possible or that Sector A should overflow. Nonetheless, this principle applies and, using common sense, this is not an invitation for a bazaar.
Therefore, it should be noted that the more extreme the condition you impose on the previous sector, the sooner you should tell them - if possible. It's all a matter of communication and pre-planning.
Use your head and common sense, think outside the box and communicate reasonably and realistically with each other.