The ASTERIX standard defines the smallest piece of metadata information:
Data Item: The smallest unit of information in each Data Category.
And the implementation equivalent:
Data Field: Physical implementation for the purpose of communication of a Data Item, it is associated with a unique Field Reference Number and is the smallest unit of transmitted information.
But, neither of them are atomic. The most common example is Data Source Identifier (SIC/SAC) which contains two pieces of information: SIC and SAC. For instance, such a Data Item is defined in the 048 Category:
Data Item I048/010, Data Source Identifier Identification of the radar station from which the data are received. Two-octet fixed length Data Item.
bits-16/9 (SAC) System Area Code
bits-8/1 (SIC) System Identification Code
So my question is: what is an atomic (minimal) piece of information called in the ASTERIX standard?