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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:

AAlwaysAttribute is mandatory and must have a valid value.
OOptionalAttribute is only present if a valid value is available.
NSNot Specified

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':

iInformationalAttribute is intended to be human interpretable.
aAutomationOne or more applications depend on the value of this attribute.
sStructuralThe value defines the relations in the data-model. Both human and computer interpretable.

4.3 Data types