A unique key is a single unique label in one dimension of a SpectrumType. SynchroScope requires a unique key in order to know what the default labels of spins are in the two dimensions of the 2D spectrum displayed in the Plane panel. The unique key is also needed by SynchroScope in order to determine which dimension of the 3D spectrum selected in the Strip panel matches up with the corresponding dimension of the 2D.
Unique keys are not used by the other scopes. This approach for dimension-matching and display of labels has been abandoned in the other scopes in favor of a more general method. The successor to SynchroScope is PolyScope which allows any number of labels (or none). The available labels are determined in PolyScope using Pathway Simulation with the SpectrumType and available SpinSystemType or ResidueType of the selected SpinSystem?.