NFlow.exportfmu generates the model to C through the shared code-generation pipeline (same gates and diagnostics as nflow_codegenerate, interpreter passes included, so linear acausal islands export too), wraps it with an FMI 3.0 Co-Simulation interface, and packages modelDescription.xml plus the C sources into a <model>.fmu source FMU.
External label sources become FMU inputs and external label sinks become FMU outputs. Only Float64 signals are supported at the FMU boundary; conditional-execution constructs beyond the lowered gates and FMU / nelsonFunction blocks are rejected with a typed message.