Cloud cover lies. Real drying comes from dry, thirsty air plus wind plus sun. Dryline reads the full forecast and tells you which days will actually dry your washing outside, and when to hang it out.
Drying is evaporation, and evaporation is driven by how much more moisture the air can still absorb, not by whether the sky looks grey. The core number is the vapour pressure deficit (VPD): the gap between how much water the air could hold at the current temperature and how much it already holds.
Per hour it computes saturation vapour pressure es = 0.6108·e^(17.27T/(T+237.3)) kPa, then VPD = es·(1 − RH/100). A warm day at 40% humidity has roughly 10× the VPD of a cold 93% morning, so it dries about 10× faster even under the same cloud.
That VPD is multiplied by a wind factor (1 + 0.3·u) with u in m/s (moving air sweeps away the saturated layer next to the fabric) and a solar factor (0.5 + 0.5·radiation/600) (sun heats the fabric above air temperature). The result is an hourly drying rate.
Each day it finds the longest rain-free daylight window, adds up the drying rate across it, and compares the total to a load budget (light 6, normal 10, heavy 16 rate-hours). If the total clears the budget the load dries; the verdict scales with how much it clears it by. The recommended hang-out time is when drying first becomes productive, and the dry-by time is when the running total reaches the budget.
Hours where rain is likely (per your caution setting) are blocked entirely, because rained-on washing is the whole problem you are trying to avoid.