Dimension
An attribute used to break a metric down, such as country, device, campaign or landing page.
What makes it wrong.
The mechanism, not the warning.
Adding a dimension changes totals, and not only their breakdown. Rows too small to report are withheld under thresholding, so a broken-down report can sum to less than the same report without the breakdown, with nothing indicating the difference.
In detail.
Metrics are the numbers, dimensions are the ways of cutting them. A report is a choice of both, and not every pair is valid: platforms maintain compatibility rules, and an incompatible request is refused rather than approximated.
The more dimensions applied at once, the smaller each resulting group, which is what eventually triggers sampling or thresholding.
Through an agent.
What changes when a model asks instead of a person.
Compatibility matrices are exactly the kind of knowledge a model cannot infer from an error message. Servers that carry the matrix can refuse an impossible combination before spending the call.
Where this is measured.
Dimension comes from Google Analytics 4. The server page lists every tool that returns it, with its parameters.