IT System description requires 3 concepts
A Software Service is a callable piece of software defined by an Interface (what the caller must know) and an implementation (hidden part which does the work).
When a Function is computerized, it is implemented as a Software Service.
A Software Service may call other Software Services which embed part of the complexity.
The Software Services are grouped into Classes
CEISAR suggests designing a System using Object Oriented Approach even if the implementation is done through classical technologies. A Class is the logical definition of Objects sharing common Attributes (or Data) and Methods (or Software Services). An Entity is implemented with one or several Classes.
A Block is a set of Blocks or Classes. As for any complex system, like a Town, System simplification comes from hierarchical decomposition into independent Blocks.
Ex: Accounting Block, CRM Block, Reference data Block, …
The structure of Blocks + their interfaces is called “Block Cartography”.
The objective is to build a Block hierarchy which minimizes exchanges between Blocks.