x1 is the action (physics quantity) of system x2 during the course of evolution/history/over path/within spacetime region x3 under conditions x4
x1 should be filled by a dimensionful real number; that is, it should be filled by a word for "Joule-seconds" (or a unit with the same dimensionality) which then has its 'amount' terbri (usually the second one) accessed by "be" and filled with a real number. In literature, the symbol that represents action should be visually similar to the symbol that represents entropy, for deep physical reasons.