The meant that if a mare was further back in the scene (or a different panel in the case of comics), you'd lose out on images that feature a stallion's plot but happen to have a mare somewhere in it. And in your previous search, you could do this instead for any results for the both of them like so:
@(stallion, butt, -mare) OR (stallion, butt, solo)@ Granted, the former would already include the latter. But it'd also catch any images missing the solo tag as an upside I guess.
That being said
"@Background Pony #9DAD":/forums/meta/topics/philomena-proposal-for-enhanced-tagging?post_id=1370#post_1370
>*Changes to the tag box?* etc.
That tagging would be WAY too far for me to even _want_ to bother. As potentially useful as that is, even e621 doesn't do that shit, and they're autists into tagging as far as they already do.