This leads to groupings which are referred to as Message Categories. Message Categories are structured in a hierarchical way, with the more generally applicable categories being at the tiers of the hierarchy with lower numbers. A category which is derived from another category (i.e. in a tier with a higher number) is called a subordinate Message Category. A category from which another category is derived (i.e. in a tier with a lower number) is called a superordinate Message Category. Figure 5.3 summarises the hierarchy.
Note that the ‘Command’ part of the hierarchy covers requirements for both the Command and the corresponding Response. Except in certain error cases (e.g. cryptographic processing failure), a Command always leads to a Response.
Section 6 is structured according to the hierarchy at Figure 5.3.