Creo, Intellego, Muto, Perdo, Rego-Animal affects "all natural living things that are not plants or humans, doing to animals what Mentem and Corpus spells do to people" as well as "things made with animal products" such as leather, wool, cheese, silk, etc. Since bacteria were unknown in medieval times, illness (e.g.) was considered either a form of possession or an imbalance of 'bodily humors'; thus, magic dealing with disease is relegated variously to Creo, Mentem and/or Vim effects.
Creo, Intellego, Muto, Perdo, Rego-Auram affects lightning, wind and gaseous substances; other weather effects typically require an Aquam requisite (see below).
Creo, Intellego, Muto, Perdo, Rego-Aquam is used for any liquid, with the exception of blood (which requires Animal or Corpus magic to affect); non-liquid forms of water will involve requisites (see below).
Creo, Intellego, Muto, Perdo, Rego-Corpus (the incorrect declension Corporem was used in older editions) applies to the human body, making it crucial to longevity formulas.
Creo, Intellego, Muto, Perdo, Rego-Herbam primarily involves plants, but applies equally to any organic matter, living or dead, that is not of animal origin.
Creo, Intellego, Muto, Perdo, Rego-Ignem involves light and heat, and is heavily represented in the fire spells of House Flambeau.
Creo, Intellego, Muto, Perdo, Rego-Imaginem (previously Imagonem) deals with images, sounds, and other sensory stimuli (thus is involved in most illusionary effects).
Creo, Intellego, Muto, Perdo, Rego-Mentem deals with emotions, memories, thoughts and spirits.
Creo, Intellego, Muto, Perdo, Rego-Terram involves earth and minerals: mere soil is the simplest target, while stone, metal and gems require progressively greater investment of spell levels to achieve the same effect.
Creo, Intellego, Muto, Perdo, Rego-Vim ("power") involves magic itself, as well as demons (the overlap is not widely understood, but the fact that there is one is a significant obstacle to the Order's 'public relations', particularly concerning the Church).
I guess you get where I'm going with this:D?