How about "hierarchical relation"?
E.g.
A Model of Workflow
Functional decomposition
In summary, the decomposition process described above leads to hierarchical ordering of the functions and sub-functions represented on the IDEFO diagrams.
http://dcom.arch.gatech.edu/cs6754/Readings/vegasurvey.pdf
Reference to "decomposition relation" would appear to be found only in material not originally written in English, e.g.:
The Road to a Business Process Architecture
In this case, business processes are organized according to the functions to which they belong. Secondly, functions and processes can both be organized into hierarchical structures through decomposition relations, which should be closely aligned.
There is no consensus about all the possible relations that can exist between business processes. However, the following types of relations are frequently used throughout the literature.
decomposition relation
specialization relation
trigger relation
use relation
http://cms.ieis.tue.nl/Beta/Files/WorkingPapers/wp_350.pdf