What a parameter is
A parameter is a single user-controlled value — a number, date, string or boolean. On its own it does nothing. It only has an effect when referenced by a calculation, filter, reference line, or set.
That indirection is the point: one control can drive several different behaviours at once.
Parameters vs. filters
A filter reads its values from the data and restricts rows directly. A parameter holds a value you define and does nothing until something references it.
Consequences: parameters don't update when the data changes (a new region won't appear automatically), and classic parameters accept only a single value. Filters can be multi-select and are data-aware. Newer Tableau versions add dynamic parameters that refresh their list on workbook open, which closes part of that gap.
Dynamic top N
Create an integer parameter Top N with a range of 5 to 50. On the Filters shelf, set a Top filter by field and choose the parameter instead of a fixed number.
Show the parameter control and users can dial the list length themselves — one control, no extra sheets.
Measure swap
Create a string parameter listing Sales, Profit, Quantity. Then write:
CASE [Select Measure] WHEN 'Sales' THEN SUM([Sales]) WHEN 'Profit' THEN SUM([Profit]) ELSE SUM([Quantity]) END
Put that calculation on Rows. One chart now serves three metrics. The same trick with a dimension-returning CASE gives you a dimension swap, which is how you build a single chart that pivots between Category, Region and Segment.