Mixing : Basic Mixing : Sends : Send Path Selector Choices

Send Path Selector Choices
The choices available in track Send selectors include bus and output paths.
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The names, format, and channel mapping of busses and output paths can be customized in the I/O Setup dialog. See Output Busses.
Internal Mix Busses
Pro Tools provides 256 internal mix busses for routing signals internally. Internal bus paths are useful for submixing and processing with plug-ins. Bus paths are defined in the I/O Setup dialog, and are available in all supported channel formats (mono, stereo, or multichannel, as supported on your system).
Hardware Outputs
Hardware sends are often used for headphone cue mixes, or for sending signals to external effects processors. Sends do not automatically return audio as do hardware inserts.