In accounting, relevant range refers to a limited span of volume or activity. To illustrate, let's assume that a manufacturer's monthly production volume is consistently between 10,000 and 13,000 units and between 20,000 and 25,000 machine hours. Within this range of activity it operates smoothly with the same amount of monthly fixed costs (say $200,000) for supervisors, rent, depreciation, etc. If the volume were to drop below this range, the company would reduce the number of supervisors, the space rented, etc. so that its total monthly fixed costs would be smaller. If the volume exceeds the range, the company would incur additional fixed costs for more supervisors, space, etc. Hence, this company's relevant range of activity is 10,000 to 13,000 units of product or 20,000 to 25,000 machine hours. It is only in this relevant range that the monthly fixed costs are $200,000.
There are also relevant ranges for variable costs and selling prices. Volume that is lower and/or higher than the respective relevant range can mean different variable costs per unit and different selling prices per unit.
In short, cost behavior and pricing is complicated. In order to simplify the analysis accountants will often assume that costs and selling prices are linear within the narrow band of activity known as the relevant range.