The CFA (Climate and Forecast Aggregation) conventions describe how a netCDF file can be used to describe a dataset distributed across multiple other data files. A CFA-compliant aggregation can be described in netCDF in such way that the describing file does not contain the data of selected variables ("aggregation variables"), rather it contains variables with special attributes that provide instructions on how to create the aggregated variable data as an aggregation of data from other sources, each of which may be self-describing datasets in their own right.
In general, netCDF variables always contain their own data and dimensions. An aggregation variable, however, does not contain its data—and therefore nor its dimensions—in the usual manner and yet still needs to be viewable as if it were a usual netCDF variable. This is achieved by encoding the aggregation variable as a scalar variable (with arbitrary single value) and providing extra variable attributes from which the variable's true dimensionality can be inferred, and the variable's aggregated data can be constructed.
The CFA conventions only apply to the data definition of selected variables, so the CFA conventions have been designed to work alongside the CF (Climate and Forecast) conventions that specify the geophysical meaning of all variables in the file, whether their data are defined as aggregations or not. The CFA conventions do not duplicate, extend, nor re-define any of the metadata elements defined by the CF conventions. However, when CF-compliant software is used for reading the discovery metadata of a CFA-netCDF file, with no expectation of reading the data of aggregated variables, a small extension is needed to allow the correct interpretation of the dimensionality of aggregation variables.