OrCAD: Uniqueness/Scope of Net Aliases, Busses, and Net Groups, Hierarchical Ports, and Off-page Connectors
I made a bunch of tests with a fake project and PCB layout.


Net aliases are unique only within the pages they appear in.
Aliases for off-page connectors are unique only within the schematic they appear under.
Hierarchical ports are only unique under the schematic in which
they appear. If they were not they would be unable to serve their purpose with re-usable
hierarchical blocks where block representations of one schematic are placed within another schematic. I have always used them instead of off-page
connectors because they look better but it looks like this is ultimately a bad idea once you start going into hierarchical designs.
When defined, a netgroup is a template for the entire project. Then different instances of it can be placed throughout the schematics. Busses, and instances of netgroups are unique only within the page that they appear. Off-page connectors are required bring them off page.


Also, a bunch of additional tests determined that:


Within a page, the net alias for entries into a line going to a hierarchical port for a netgroup must share the same net alias. If no
net alias is given for the line it will adopt the name of the
hierarchical port to which it is connected. (I did not check how it
behaves if the line is given a different alias than the port name).
A line connecting two hierarchical ports must be named
Individual connections between two hierarchical ports for netgroups are determined by the order of signals defined in definition of both netgroups (1st to 1st, 2nd to 2nd, etc).
Individual connections between two hierarchical ports for busses (and presumably component pins defined as busses) are made based on the wire entry tag numbers (i.e. not according to any placement ordering that might appear on the schematic)
Connecting two busses contained in a hierarchical block does not require naming of the bus. This appears to be the one exception where a multi-connection line (i.e. a bus or netgroup line) does not require a name.
Presumably, another instance where a bus does not require an alias is if there are wire entries entering and exiting the bus on the same page that share the same net alias. That said, if the bus is then giving a name that conflicts with the net alias for the wire entries, I suspect the connection would be broken.