hi,
in my view eTOM works with three steps:
(1)Process collection: It deals with the requirement of all the process collection which are there to run the system or the organisation.
(2)Process Classification: This is associated with the grouping of processes or classification of processes to various sub-processes.This helps to focus more on single type of problem and tackling them simultaneously.
(3)Process Decomposition: This is the branching of one group processes to various sub processes,its like Tree and its Branches.Stem is the main root and here its main process the branches depends on the priority level and the importance of the process.
So, the whole decomposition process is actually accumulating all the processes and again braching it .
Regards,
Ashutosh