Lots of popular BDSM resources use the word Switch to mean two totally different things. This was actually popularized by TNG because in very Traditional BDSM the identity of Switch did not exist. Let me explain.
Old Guard hierarchy derives itself from the structure of Military Rank. If you begin as a Private or a Corporal and through mastery of skill and experience reach a commissioned rank such as Captain or Major, the responsibilities of your commissioned rank prohibit you from ever going back to being a Private or a Corporal again. In OG, Mastery was transformative. It was impossible to be the Master of one person and the slave of another because Masters became Masters through Mastery. It was perfectly acceptible for a man to choose to remain a slave and not desire to work toward Mastery.
Within Old Guard there was however a word for men who liked to give and receive kinks, men who were bottom to certain kinks and perhaps Top in other kinks were called Versatile. In gay culture today this terminology is still in use. Some men are predominantly Tops, others are bottoms, and those who are some of both are Versatile.
In the early 1990’s when Old Guard faded and more modern ways of BDSM became more common is when we first see the identity of Switch begin to form. Before this time, people largely earned their titles and within modern BDSM we pick our titles based on our preferences. They are self-assigned, hopefully with our personality and actions adding validation to what we’ve chosen. The idea of people earning Dominant titles through Mastery has faded.
The identity of Switch is a person who has the potential to be the Dominant in one relationship dynamic and the submissive in another relationship dynamic. This identity was formed based on preference. Sometimes a Switch prefers one relationship at a time. Others practice being a Switch through non-monogamy.
Oftentimes BDSM resources, especially TNG-influenced resources, will confuse everyone by using the word Switch to mean being Versatile. Anyone who prefers to be the Top during some kinks and the bottom during other kinks in a scene is Versatile. This even applies to the kink of roleplaying. If one acts like a dominant in some scenes and acts like a submissive in other scenes the person is Versatile. Roleplaying is when one acts like something within a scene that does not match their identity outside of that scene.
Another TNG invention is mixing the identity of Switch and the kink of Versatile roleplaying together. Out of that mutation comes a Switch who is somehow both Dominant and submissive within the same relationship. There isn’t any real way for this to work other than causing chaos and a breakdown of what it means to be truly Dominant and truly submissive.
To have a successful D/s dynamic there must be consistency with the power exchange. The s-type gives the D-type the power to discipline and place them in mental bondage. This consistency will hold everyone accountable to maintain the agreement/contract that was negotiated and consented to. Constantly switching within the same dynamic breaks down this mindset. There is no flow of who is in charge and who is not. No rules would be taken seriously. The rules can be changed when the other person takes charge. Within a power exchange dynamic, there has to be a constant person in power to keep the discipline in order, the mindset of the mental bondage in place to be beneficial to the dynamic. The identity of a Switch is someone who is consistently the Dominant in one relationship dynamic and consistently the submissive in another relationship dynamic.
A couple that appears to be constantly switching D-type and s-type identities are actually roleplaying their Dominance and submission. Being able to dominate someone for a period of time is not the same as being a Dominant. Same with submitting to someone for a period of time and being a submissive. I could for, say a 24 hr period, dominate my Dom, be in total control within his limits. I am not the Dominant. I am role playing the Dominant. The difference is the consistency of the mindset. I have the mindset of a submissive, however I can act out or play any role that you need me to.