Stairs were the one built element of the takeoff with lumber in the order but no labor line in the budget. Stair carpenters usually quote by the riser, so laborStairRiser multiplies the riser count the layout already computed by the rate you type.
A stair that splits at a landing counts both flights. Nothing is priced unless you type a rate: with no laborStairRiser, no stair labor line appears, and like every labor line it carries no tax or contingency. The tool ships no prices.
Checked by budget.test.mjs: a 108 in stair and a 200 in landing stair give a riser count the line's quantity matches, and amount equals quantity times the rate.