Tiers
Four promises, one refusal. Every stage of every match carries exactly one of them.
Why tiers
Not every block of DSP can be matched the same way. A parametric EQ has an impulse response you can null against; a compressor with a full control surface does not. Pretending otherwise is how you end up with a plugin that sounds right on one song. Tiers make the difference explicit before any work starts.
How a stage gets its tier
The probes decide, not the agent. A sweep and a null test settle A. Two-level and programme-material probes settle B. C is declared when the surface is too large for a bounded fit. D is declared when the reference is time-varying. The overall tier of a plugin is the worst tier in its chain.