The so-dom can have a "one on one" focus too though, especially if introverted or if it makes sense for their ego fix (ie. withdrawn types). I think this type might be the kind who wants/needs just the one "best friend". They may allow themselves to be drained a bit in maintaining that relationship. They seek a connection also, but so as to "belong", and I think many confuse that with being sx-dom. They think so-dom is about carving a spot in a group, but it's just about carving a spot, period. That can very much be emotional intimacy with another.
The sx-dom has a focus on who EXCITES them though, and who they can excite. For an introvert, I imagine this is a strain, because introverts tend to devalue the external, resist it a bit, unless they can conform it to their inner world. However, you probably can see how being excited & being exciting can be a way of conforming the external
to you as opposed to allowing yourself to be affected a lot. IMO, the sx-dom introvert weeds out a lot, and may mercilessly ignore anyone who doesn't EXCITE so as to focus energy on those few who do. This kind of exclusion is different from the kind an so might partake in...because they will exclude people too. Sp-dom discriminate the least; we disdain & avoid everyone equally (har har).
It seems to me sx-dom can end up in relationships with people they don't even really like, just because that person excites them & vice versa. It's very "moth to flame".
To contrast the so-dom again: they tend to ignore & avoid those who don't provide a comfort, that feeling of belonging. When you have a person who is sx-last, then, they may tend to also avoid those who excite them, because it's unstable & a threat. I notice these people tend to choose relationships with those who feel "comfortable" to them. To someone who is not sx-last, it's almost inconceivable to not want to be very excited, however uncomfortable it can be. However, sx last may form relationships with people who are truly compatible with them based on the important stuff, not just "chemistry"; but they may also miss out on potential connections when they avoid someone because the excitement is "too much".
We hear a lot about the conflict of being sx + sp, but I think there is a conflict with sx + so too. I think that conflict is between "comfort & belonging" with others & "excitement" with them. I think the people who have "frenemies" are often this combo. When you are so last, that is the weirdest thing ever to see

. I understand the occasional draw to someone who riles you up, but to need to slap a "we're actually buddies" label on it is bizarre.
Of course, sp-dom are the least people-oriented, which is why introverts may confuse themselves for one when they are not one. Being sp-dom is something like "self-soothing". You excite yourself, you belong to & comfort yourself, and you preserve yourself by making the factors controllable to you (and people generally are not). This ranges from security-seeking to decadence & indulgence, depending on the ego fix.
Basically, you have to see the instinctual stack in context of the ego fix (which you could say is enneagram + Jungian type, at least).