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Forward data for ports that are both internally and externally connected. e.g. click-io may be connected to a track as well as physical outputs. Since the port is externally connected, data is written to an internal buffer _data[], and only at cycle-end resampled to engine-buffers. However a track's recording the metronome is not externally connected. Hence data is directly read from the engine-buffers. Summing also happens at engine level, so data has to be written back for downstream ports to read them in the same cycle.
Please see the Ardour web site at https://ardour.org/ for all documentation..
For information on building ardour:
https://ardour.org/development.html