https://symfony.com/doc/current/reference/constraints.html groups constraints into categories. But the collection constraints category looks confusing to me. I don't understand what "collection" means in this title:

• Locale, Language and Country validate the value as belonging to a given set of values
• Choice does a similar job, but additionally supports being used on an array when using its multiple option (which is not exactly equivalent to using @All(@Choice(multiple=false)), as it reports only the first invalid value when using multiple=true and does not support Traversable)
• Count applies on an array or Countable value (so the "collection" concept here might be the validated value)
• UniqueEntity is a class-level constraint validating uniqueness in the DB. I don't see any concept of collection here
• Collection is about applying constraints on specific keys of an array, for cases where you use it as a struct (what Hack calls a shape), and precisely not when treating the array as a collection

And All is about applying the same constraints on all items of an iterable (useful to validate collections of items) and is not part of that category.