As in Ying and Yang, stuy of 8-words has an anatomy of what's visible and what's not. For instances, h2o that's visible to the nakes eyes is 壬, and the vapor in the air which cann't be take account to the naked eyes is 癸; fire you can see is 丙, and 丁 (heat) is the fire that you can't see; metal, earth, and wood, etc, etc, etc.

Regarding money, 正財 is the money you can take account on, like your money in your deposit. 偏財 is other people's money, like parents' money (which is out in the open to 比 and 劫, sister and brother).