The relevant range of activity refers to a the current level of production. If production drops or increases, then the relevant range will change.

outside the relevant range, variable cost and fixed cost behaviors patterns may change

an increase or decrease on a company's fixed costs is however not only dependent on the relevant period but also on the relevant production range. The total fixed costs will remain constant if the relevant production range can be handled by the same number of production units, producing fewer steps. If a certain step ( certain cost level) encompasses the entire relevant range of activity, the costs are entirely fixed.

The relevant range of operations.

The price range that an asset or commodity will fluctuate within. The relevant cost range for a barrel of oil has been increasing dramatically thanks to the US Biden administrations policies and the Ukraine - Russia war, for instance.