What is a context filter?

A context filter is a filter promoted above the other dimension filters, so that everything applied afterwards operates on its result.

Tableau creates a temporary table containing only the rows that pass it. Subsequent filters, top-N rules and FIXED expressions all see that reduced set rather than the full data.

The main use is making a top-N filter behave sensibly alongside another filter. Without context, both are ordinary dimension filters and the top 10 is computed across everything before the second filter narrows it.

Set it by right-clicking the pill on the Filters shelf and choosing Add to Context — the pill turns grey. Bear in mind the temporary table is rebuilt whenever the context filter changes, so it is not free.