What FAR clauses are there that pertain to past performance? In contracts, what is the difference between the terms Shall and May? Which circumstances and clauses allow the Government to allow a rebuttal at their discretion? Which clauses and circumstances requires them to allow a rebuttal?

Does it matter where in the review process that they are at? Does the type of proposal dictate whether or not the Government must allow for a rebuttal? Does it matter where the negative performance report was pulled from?