Definition
In the project charter, cost, time, and performance are each ranked as "least flexible / middle / most flexible" (the card and rule pack uniformly call this the flexibility matrix); ties are not allowed—a tie means no ranking.
In detail
The flexibility matrix answers one question: when three things must sacrifice one, which is sacrificed first. Cost, time, and performance each get one tier; each row and column can have only one check. The original explicitly states no ties, because a matrix with ties gives no answer when real trade-offs are needed. Question 2 of the sponsor's 17 questions, "What is the least flexible constraint?," refers to this matrix.
Example in practice
The consulting project's final submission date is a hard deadline, so time is set to "least flexible"; there is no consulting fee but hours are capped, so cost is set to "middle"; the task brief itself states the scope can be prioritized by time, so performance is set to "most flexible". The system verdict passes; if both time and cost are set to "least flexible", it will be judged "tied equals no ranking".