Define parameters in Tableau and their working.

A parameter is a single user-controlled value — a number, date, string or boolean — that sits outside the data.

On its own it does nothing. A parameter only has an effect once something references it: a calculated field, a filter, a reference line, or a set. That indirection is the point, because one control can then drive several behaviours at once.

The usual pattern is to create the parameter, show its control, and reference it in a calculation. A string parameter listing `Sales`, `Profit` and `Quantity`, referenced by a `CASE` expression, lets one chart display any of the three.

Newer versions add dynamic parameters, which refresh their list of values when the workbook opens. Classic parameters do not — a new region appearing in the data will not show up in the control until someone edits it.