Two ways to write the same data
Block style — the YAML look
Lists use - with a space, one item per line. Maps use key: value, one pair per line. Indentation defines nesting. This is what 95% of YAML in the wild looks like.
Flow style — the JSON look
Lists use [a, b, c]. Maps use {key: value, key: value}. Same data, JSON-shaped. Useful for short inline values where block style is overkill (a single-line list, a small map).
Mixing block and flow
You can put flow inside block. tags: [ai, daughter, engineer] is a one-line list inside a larger block document. The reverse — block inside flow — is technically allowed but reads poorly.
Principle: use block style for anything that benefits from line-by-line review (lists of dependencies, env-var maps, multi-step CI jobs). Use flow style only for short inline collections where the block form would just add noise. The 'compact JSON inside YAML' aesthetic is a smell.