It's a choice. Any Guardian can learn to use any subclass. In fact, any Guardian can learn to use powers of any class. A Titan could theoretically create a Golden Gun, a Hunter could shoulder charge, and a Warlock could create a Ward of Dawn. There's nothing barring them from learning these things. The difference between classes and also subclasses is largely a cultural one and is dependent on personality. A Guardian who is curious, daring, and crafty would most likely find it easier to imbue a knife/staff with arc light or a gun with solar light. They would enjoy patrols and adventures beyond the wall much more than studying or training. They would likely befriend others like them. Therefore, that specific Guardian would be molded into the Hunter archetype. Titan-like Guardians would likely join the Titan Order of their liking. Would-be-Warlocks would join the Warlock Order that interests them most. Because of that, they'd learn the subclass path most akin to that order. The Praxic Fire would teach upcoming Warlocks the Sunsinger and Dawnblade paths. The Firebreak order would likely teach the Striker path. Hunters would learn on their own or from word of mouth/other Hunters. Not to mention Pahanin's Errata. Before the City became organized and before Guardians became institutionalized, Guardians/Risen didn't follow strict paths. It was only after social groups and cliques of Guardians arose did classes become a thing. Subclasses can be switched on the spot, but each Guardian has their preference.