The right-hand side of each equation representing a version of Little's law is written vertically in the order $R, W, S$, which matches the expression $R=W+S$ for the mean residence time, viz., the sum of the mean waiting time ($W$) and the mean service time ($S$).
The letters on the left-hand side, $Q, L, U$ (reading vertically) respectively correspond to the queueing metrics: mean queue-length, mean waiting-line length, and mean utilization. Those letters can also be read as the word ``clue''.
Incidentally, the middle formula is the version that appears in the title of John Little's original paper:
J. D. C. Little, ``A Proof for the Queuing Formula: L = λ W,''
Operations Research, 9 (3): 383—387 (1961)