Auto subs are subs made during the match.
Auto player as questioned is before the match kicks off inorder to replace illegible players.

I think the reason why auto players are chosen from the reserves, as first choice, is so as not to disrupt the sub settings.
I always look at opp reverses first when playing against red carded players in line up. Opp injuries feedback I get from previous game review, if possible.
If no reserves, I look at subs list for possible replacement strengths and weaknesses.