The second component of a CDF is the metadata. Metadata values consist of user-supplied descriptive information about the CDF and the variables in the CDF by way of attributes and attribute entries. Attributes can be divided into two categories: attributes of global scope (gAttributes) and attributes of variable scope (vAttributes). gAttributes describe the CDF as a whole while vAttributes describe some property of each variable (rVariables and zVariables) in the CDF. Any number of attributes may be stored in a single CDF. Section 2.5 describes standard CDF attributes used by the NSSDC. The term ``attribute'' is used when describing a property that applies to both gAttributes and vAttributes.
gAttributes can include any information regarding the CDF and all of its variables collectively. Such descriptions could include a title for the CDF, data set documentation, or a CDF modification history. gAttributes may contain multiple entries (called gEntries). An example of this would be a modification history kept in the optional gAttribute, MODS. This attribute could be specified at CDF creation time and a gEntry made regarding creation date. Any subsequent changes made to the CDF, including additional variables, changes in min/max values, or modifications to variable values could be documented by writing additional gEntries to MODS.
vAttributes further describe the individual variables and their values. Examples of vAttributes would include such things as a field name for the variable, the valid minimum and maximum, the units in which the variable data values are stored, the format in which the data values are to be displayed, a fill value for errant or missing data, and a description of the expected order of data values: increasing or decreasing (monotonicity). The entries of a vAttribute correspond to the variables in the CDF. Each rEntry corresponds to an rVariable and each zEntry corresponds to a zVariable. Sample vAttribute rEntries for the Temperature rVariable from the example above are shown in Table 1.6.
The term ``entry'' is used when describing a property that applies to gEntries, rEntries, and zEntries.
Table 1.6: vAttribute rEntries for the Temperature rVariable