Before getting into details, it is important to understand some basic terms used when talking about space usage on the appliance.
Physical Data - Size of data as stored physically on disk. Typically, this is equivalent to the logical size of the corresponding data, but can be different in the phase of compression or other factors. This includes the space of the active share as well as all snapshots. Space accounting is generally enforced and managed based on physical space.
Logical Data - The amount of space logically consumed by a filesystem. This does not factor into compression, and can be viewed as the theoretical upper bound on the amount of space consumed by the filesystem. Copying the filesystem to another appliance using a different compression algorithm will not consume more than this amount. This statistic is not explicitly exported and can generally only be computed by taking the amount of physical space consumed and multiplying by the current compression ratio.
Referenced Data - This represents the total amount of space referenced by the active share, independent of any snapshots. This is the amount of space that the share would consume should all snapshots be destroyed. This is also the amount of data that is directly manageable by the user over the data protocols.
Snapshot Data - This represents the total amount of data currently held by all snapshots of the share. This is the amount of space that would be free should all snapshots be destroyed.
Quota - A quota represents a limit on the amount of space that can be consumed by any particular entity. This can be based on filesystem, project, user, or group, and is independent of any current space usage.
Reservation - A reservation represents a guarantee of space for a particular project or filesystem. This takes available space away from the rest of the pool without increasing the actual space consumed by the filesystem. This setting cannot be applied to users and groups. The traditional notion of a statically sized filesystem can be created by setting a quota and reservation to the same value.