The requirements for each type of reserve depend on the uncertainty margins in scheduled production and estimated demand at various timescales. The way the requirements for reserves are calculated is based on statistical principles (see box page 36: Strains on the System).
"Regulating reserve" actually comprises several types of generation plant. "Frequency response" plant act automatically, increasing or decreasing output in response to changes in system frequency. A fall in frequency means demand exceeds supply, so more power is needed. A rise in frequency indicates supply is exceeding demand. "Spinning reserve," as the name implies, is operational, but at less than full output. "Standing reserve" is not generating, but ready to do so.