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?