# Meta-data attributes

Meta-data associated with a dataset/file is represented using attribute/value pairs. The set of available attributes is restricted. Meta-data attributes are classified into four categories:

- `System-defined attributes`: e.g. size, checksum, creationtime, modificationtime, status
- `Physics attributes`: e.g. like number of events, cross-section, or GUID
- `Production attributes`: storing information like which task or job produced the file
- `Data management attributes`: necessary for the organisation of data on the grid (see Replica Management section)

For datasets, it is possible that the value of a meta-data attribute is a function of the meta-data of its constituents, e.g. the total size is the sum of the sizes of the constituents. In this case it is obviously not possible to assign a value to it.