unique pid [EtherGuide Glossary]
The use of a packet id is said to be a unique pid when the listed use is the only one permitted of that packet id in a transport stream.
For example, Program Association tables are the only use permitted of packet id =0, so this use is considered unique. Program Map table sections for different program_numbers can share the same pid, and that pid can be used for other purposes, so that use is not unique.
Many PSIP table sections use the pid 8187 / 0x1FFB, so the use is not unique.