System Configuration : I/O Setup : Valid Paths and Requirements

Valid Paths and Requirements
While configuring the I/O Setup, certain rules apply for path definition and channel assignment. All paths must be valid before the I/O Setup configuration can be applied.
Though it is possible to set up invalid assignments in the Channel Grid, Pro Tools will not accept an I/O Setup configuration unless all paths meet the path definition and channel assignment requirements, as follows:
Minimum Path Definitions
All paths must have a name, be of a specific format, and have a valid I/O assignment.
Valid Paths
Channel assignment follows certain rules regarding overlapping paths.
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A newly-created output path must either be completely independent of other assignments (not assigned to any other available I/O interface/channels), or it must be a path completely contained within a larger path (for example, an LCR sub-path within a larger 5.1 path).