Out adds it's output to a given bus, making it available to all nodes later in the node tree (See Synth and Order of execution for more information). ReplaceOut overwrites those contents. This can make it useful for processing.
The index of the bus to write out to. The lowest numbers are written to the audio hardware.
An Array of channels or single output to write out. You cannot change the size of this once a SynthDef has been built.