Hi everyone, I am new to observations and observational terms! I am reading the paper "constraining the time variation of the fine-structure constant" by Srianand et. al in the section "constraining alpha with QSO absorption lines" there is a sentence saying "... rest wavelengths of MG II .... thereby providing a good anchor for measuring the systematic redshift." - What does the systematic redshift mean? - What do they use Mg II for?