Which S parameter represents input port return loss or reflection coefficient (equivalent to VSWR)?
Why S parameters are written as S(receiving port)(driving port), so the first subscript is where the signal is measured and the second is where it was fed in. S11 means drive port 1 and measure what comes back out of port 1, which is the input reflection coefficient, and that is what a VNA converts into return loss or VSWR at the input. S22 is the same idea at the output port, while the mixed subscripts describe signal passing from one port to the other.
Watch out S21 is the forward transmission term, the gain or insertion loss from input to output, and S12 is the reverse transmission or isolation; neither describes reflected energy at a port.
Matching subscripts mean reflection: S11 in, S22 out. Mismatched subscripts mean transmission through the device.