I'd say it's more of a "light" tone than an unofficial tone (the actual name for it is literally that, "轻声"). Used when a character is to be read without emphasis. In a sense, you can say it's equivalent to how "-ly" works in "silently" (e.g. 悄悄的 - the 的 here uses the light tone). Also used in nouns with repeated characters, e.g. father ("爸爸" - 4th tone in first character, light tone in second character).