The state where a daydreamer (viewer or reader) imagine themselves as the character of a story, often a favourite one at that. Like I said, sometimes accusing characters of being author surrogates’s moot since a number of fictions are already semi-autobiographical to an extent, whatever extent. (An author might admit that part of his fictions are based on his personal trauma for instance.) Not to mention Canon Sues are far likelier to be supporting characters.
Maybe not exactly but since a good number of fictions and written works in general can/tend to be semi-autobiographical, with authors sometimes basing characters and stories after personal incidents in whatever manner so keep in mind most author surrogates aren’t (necessarily) Mary Sues and generally aren’t. The real Canon Sues are usually likelier to be supporting characters, the deliberate audience surrogates if you will.
Maybe not necessarily exactly but it’s parsimonious that audience surrogates aren’t necessarily the main protagonists (or main focus) most of the time. They’re blank slates enough to be made into Mary Sues whilst when it comes to the protagonists there’s a lot we know about their struggles, flaws and virtues. Maybe not necessarily or always but they do deserve to be in the spotlight.
Keep in mind supporting characters aren’t always Canon Sue and whilst a good number of supporting characters are and can be author surrogates, they too aren’t necessarily Mary Sues either. But an audience surrogate’s often likely to be a supporting character. A spectator against the main character whom everybody else are supposed to observe from the outside.
Audience surrogates are thus likelier to be co-opted as fanfiction Mary Sues. Consider what’s been done to Willow of Buffy fame or more recently Felicity Smoak from Arrow. If you want a good example of a male counterpart, look no further than Tim Drake and Dick Grayson. As in these two were purposely created as audience surrogates and more often than not (heck even moreso than Batman) function as power fantasies.
It wouldn’t matter how much tragedy befell them, Dick dates an alien refugee and Tim’s got a harem of sorts including a groupie whilst Batman’s an aloof jerk whose well-known girlfriend’s a stereotypical cat owner and has a bastard son (whom the audience can’t stand) with another woman. I mean they’re bigger Mary Sues than Batman will ever be under my logic.
Maybe not exactly but still.