Identifier for a kind of activity (sport, fitness modality, mind-body practice). Encoded as a snake_case, dot-separated string. Each .-separated segment matches [a-z][a-z0-9_]*; segments express variant hierarchy from general to specific.

Examples:

• yoga, tennis, basketball — single-segment activities
• pilates.reformer, pilates.mat — variants of pilates
• skiing.cross_country, skiing.downhill — variants of skiing
• volleyball.beach — beach volleyball (indoor stays as volleyball)
• martial_arts.mixed — MMA (martial_arts is the generic category)
• other.fitness, other.sport — generic fallback subcategories

The full set of valid values is defined server-side; submitting an unrecognised identifier returns a validation error.