from the data sheet:
Reference plane extension (offset) allows you to shift the measurement reference plane away from the point established during calibration. This is useful in removing the path length of assumed ideal interconnecting , connectors cables or microstrip lines from measurements. PicoVNA 1 software allows independent reference plane extensions
on each of the measurement parameters (S11, S22, S12 or S21), either as an automatic re-reference or by manual entry. Independent extensions allow, for example, different extensions on the two ports for S11 and S22 and then thru-line normalization for S21 and S12 transmission comparison with equivalent length thru-line.
is this what you are looking for?
Thanks