Past is short for Past Level.
The Past can be any reference point of interest. It can be used to express a Product Function, Product Quality Level, development resource, or solution of an old or existing system.
The most common use of the Past Level is to reference how well a previous Product did along the same Scale quantifier.
It can be essential to know where along the Scale quantifier we and our Stakeholders are (Past Level), Before we decide where we want to be in the future (Goal Level). Specifying the Past Level gives that Benchmark and communicates it clearly. The Pest Level is not a Requirement in-itself, it outlines the development challenge, to move from where we are, the Past Level, to where we want to be, the Goal Level.