Any HDF tree node item can have attributes. Attributes can have optional values or be optional themselves. The list of attributes per HDF tree node item specifies these cases as described in the sections below.
Attributes can be mandatory or optional. This is specified in the column 'Exists?' as follows:
|A||Always||Attribute is mandatory and must have a valid value.|
|O||Optional||Attribute is only present if a valid value is available.|
Attribute is mandatory but may contain the string value 'not specified' if no valid value is available.
Note: Although HDF supports a null dataspace for scalars and vectors it is not used because it lacks the ability of specifying why it has no value.
Attributes play a certain role. Currently the following roles are recognized in column 'Role':
|i||Informational||Attribute is intended to be human interpretable.|
|a||Automation||One or more applications depend on the value of this attribute.|
|s||Structural||The value defines the relations in the data-model. Both human and computer interpretable.|