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- the same horizontal shape as another airspace but with different vertical limits ("above-below" association).
- a composition by aggregation of airspaces (e.g. union and subtraction operations).
In the first case, only one parent can be used (but it may be used for more than one child) and the derivation process is limited to the horizontal border.
In the second case, a combination of "parent" airspaces (one to many relationship) will be used. The derivation process is extended to a total aggregation of airspace volumes (i.e. may also take vertical limits into account).
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