Why S parameters (scattering parameters) describe how RF energy is reflected from and transmitted through a network, and each one is labeled with two port numbers. The convention is S(out,in): the first subscript is the port where the response is measured and the second is the port where the signal is applied. So S11 is the reflection back out of port 1 when port 1 is driven (related to SWR and return loss), and S21 is what comes out of port 2 when port 1 is driven (gain or insertion loss).
Watch out The choices about timing, data quality, or frequency order sound like instrument bookkeeping, but S parameters are frequency-by-frequency ratios of wave amplitudes at specific ports, not a record of when or in what order measurements were taken.
S(out,in): second number is where you push, first is where you look. S11 = reflection, S21 = forward gain.