Let me begin by stating up front that the following is just my opinion of what the Summit Wrestling Alliance is. It carries no weight of law or any such thing; it is just my feelings on the subject.
The SWA is a subculture. It is a subset of the larger world of e-wrestling and is made up of a group of players, characters and federations that share a common history and set of assumptions about how e-wrestling ought to work. To my mind, that's it. Nothing more, nothing less.
Let me give some history.
Way back in November of 1990, Ben Pierce started the EMWA and what would eventually be known as the SWA. Within a year or so, a couple more feds had started -- the PWGeA and the WeWF come to mind -- with a lot of crossover among players. Now keep in mind this was in the early days of the Internet, comparatively speaking, and so as far as we could tell we were the whole of e-wrestling. (In point of fact, we were the whole of "e-wrestling" since the term originated with Scott Baxter. Doubtless other people were doing PBM wrestling, and some of it over computer networks, but the term was ours.)
Then came the great influx of commercial networks onto the Internet, notable among the early arrivals was AOL. With that came a boom in the number and variety of efeds. Until that point, when we said "e-wrestling" we meant, with some accuracy, a fairly small group. After that point, it meant something else entirely. We needed a new term, and what we came up with was "The Summit Wrestling Alliance."
The term came from the Summit of e-Wrestling, a yearly or so interfederational extravaganza with tournaments, interfederational challenges and so forth.
The SWA, therefore, is a culture, or sub-culture. Because of that, membership is rather a fuzzy concept. For me, at least, at some point I realize that a given fed is "enough" like what I see as the SWA and to my mind that is the moment they become an SWA fed. Other people have different thresholds of "enough". If all of the feds which currently agree that they are Summit Feds agree that someone else is a Summit Fed then the fed is "officially" a Summit Fed.
So what does that mean? It doesn't mean a thing. It just means that people realized that a fed was already working and playing with the other SWA feds. In practical terms, it tends to mean that other SWA feds recognize the legitimacy of the fed's titles, tend to invite them to interfederational events, and move characters easily around the circuit.
Having said all that, there are a few things that all the SWA feds have in common.
First, they aren't booked. I'm not making any judgment about the validity of booking, here, but the SWA feds don't do it.
Secondly, they tend to be run by people who have a track record as players in other SWA feds. Only in a few rare cases (I can only think of one) did an existing fed join the SWA rather than being spawned from within the SWA, as it were.
Thirdly, they use original wrestlers only.