Reach answers one question: if this token, component or divergence changes, how much of the product changes with it?
How it's measured
Count the places in code that use each token and component: the files that import a component, the stylesheets that read a custom property. A button component imported in 140 files has a reach of 140 files; a one-off colour on an admin page has a reach of one.
The same count applies to problems. A divergence inherits the reach of what it affects: a drifted fill in that button touches all 140 files at once.
Why it matters
Reach is one of the four signals in how to measure design system health, and it turns a list of findings into priorities. Without it, every divergence looks equally urgent, and teams either fix the easy ones or none. With it, the drifted button fill comes first, and the forgotten admin colour can wait.
Reach also makes a health score honest. Weighting findings by reach means a problem on every screen moves the score, and a problem on one page barely does.
Where reach misleads
File counts under-count shared shells. A component imported once in a layout that wraps every page has a reach of one file but appears on every screen. Count a layout import as reaching every route it wraps.
Generated code distorts the count in the other direction. Exclude generated folders, such as exported icons, before you count.