Two classes advertised as yoga can be almost unrecognisable to each other, one physically demanding and precisely timed, the other still and largely supported. The divergence has a traceable history rather than being a matter of branding.
The physical emphasis is recent
The postural practice that dominates modern yoga occupies a small part of the older textual tradition, which is concerned mainly with ethical discipline, breath and meditation.
The extensive systems of postures now taught were assembled largely during the twentieth century, drawing on existing practice alongside physical culture and gymnastic traditions of the period.
Recognising this makes the differences between styles easier to interpret, since they are variations on a recent synthesis rather than divergent ancient lineages.
A small number of teachers produced most branches
Most globally practised styles trace to a handful of teachers, several of them students of the same influential figure who taught different students differently.
Accounts from that period describe instruction adapted to the individual, so students who trained at different times or ages received genuinely different systems.
Each then transmitted what they had learned as a complete method, which is how one teaching lineage produced approaches that now appear incompatible.
Styles differ on which variable is fixed
Some systems fix the sequence and let the pace vary, some fix precise alignment and use props to achieve it, and some fix duration and hold positions for long periods.
Others fix the link between breath and movement and treat the sequence as flexible, which produces the continuous flowing formats.
Knowing which variable a style holds constant predicts more about a class than its name does, and it is the useful question to ask before attending.
Naming is largely unregulated
Some style names are trademarked and carry defined training requirements, while others are generic terms that any teacher may use however they wish.
Two classes with the same generic label can therefore differ enormously, depending entirely on where the teacher trained.
Teacher training requirements vary between countries and certifying bodies and change over time, so a certification indicates less about content than it appears to.
How to choose without relying on the label
The informative questions concern the class itself: how long positions are held, whether props are used, how much alignment instruction is given and how large the group is.
Class size largely determines whether individual correction is possible, which matters more for beginners and for anyone with an existing injury.
Anyone with a relevant medical condition, a recent injury or who is pregnant should tell the teacher beforehand and confirm suitability with a clinician rather than relying on a style's reputation.