The single set of functions, revenue, coverage, penetration, ranking, opportunity, risk, that every screen, the planner and the assistant call. Adding a tool gives every surface the capability at once.
Because a chart and an answer that call the same function cannot disagree. A screen with its own calculation and an assistant with another is two products that will drift.
Its name, its inputs, the definition version it used, and the rows it read. That is the provenance every answer carries.
One set of functions computes every measure, and every screen, export and assistant answer calls the same functions. A figure therefore cannot differ between two places it appears.
Coverage on the manager's view, in the weekly digest and in an answer to a typed question all come from one coverage function and agree to the unit.
A second implementation written for one report. The two drift apart and the argument about which is right begins.