Basically, "Divine," "Summoning," and "Sorcery" are tags that identify a unit as being capable of casting Divine, Summoning, and Sorcery spells, respectively. For instance, the "Sun II" promotion grants "Whitefire" (Divine), "Summon Sand Lion" (Summoning), and "Blinding Light" (Sorcery). That means a divine unit with Sun II (such as a Vicar) casts "Whitefire," a Conjurer with Sun II casts "Summon Sand Lion," and a Mage with Sun II casts "Blinding Light."