  Software process lines provide a systematic approach to develop and manage
software processes. It defines a reference process containing general process
assets, whereas a well-defined customization approach allows process engineers
to create new process variants, e.g., by extending or modifying process assets.
Variability operations are an instrument to realize flexibility by explicitly
declaring required modifications, which are applied to create a procedurally
generated company-specific process. However, little is known about which
variability operations are suitable in practice. In this article, we present a
study on the feasibility of variability operations to support the development
of software process lines in the context of the V-Modell XT. We analyze which
variability operations are defined and practically used. We provide an initial
catalog of variability operations as an improvement proposal for other process
models. Our findings show that 69 variability operation types are defined
across several metamodel versions of which, however, 25 remain unused. The
found variability operations allow for systematically modifying the content of
process model elements and the process documentation, and they allow for
altering the structure of a process model and its description. Furthermore, we
also find that variability operations can help process engineers to compensate
process metamodel evolution.
