Functional decomposition corresponds to the various functional relationships as how the original complex business function was developed. It mainly focusses on how the overall functionality is developed and its interaction between various components.
Large or complex functionalities are more easily understood when broken down into pieces using functional decomposition.
Functional decomposition is mostly used during the project analysis phase in order to produce functional decomposition diagrams as part of the functional requirements document.
Functional Decomposition is done after meeting with business analysts and subject matter expertise.
Decompose the first level components with their functions and continue to decompose to lower levels until sufficient level of detail is achieved
Perform an end-to-end walk-through of the business operation and check each function to confirm that it is correct.
The below example, best describes the Functional Decomposition: