Do Coupling Data IDs match?

Is the coupling data ID the same for every participant?
If for example my fluid solver sends forces with dataID 3, should the solid solver receive forces with dataID 3 or are the dataIDs only solverinternal and don’t ‘match’?

Data and mesh IDs have arbitrary values. They can be different for different participants. Think of them as address values of pointers. There should be no use case where looking at the values should be necessary. We are currently rewriting our documentation. In the new documentation this fact will be much more pronounced:

The IDs that preCICE uses (for data fields, meshes, or vertices) have arbitrary integer values. Actually, you should never need to look at the values. The only purpose of the IDs is to talk to preCICE. You also do not look at the value of a C pointer, it is just a non-readable address. In particular, you should not assume that vertex IDs are ordered in any certain way (say from 0 to N-1) or, for example, that ‘Forces’ always have the same ID ‘2’ on all meshes.