Model-based engineering (MBE) contributes a lot to software reuse by abstracting technology independent models, their capitalization and then their reuse to produce systems, ensuring early composition and checking capabilities. One step further, it was recognized that models could be parameterized in order to capture recurrent modeling patterns. This led to the concept of “model template” as offered in UML. In this paper we concentrate on engineering practices and operators which derive from the adoption of this concept, specifically in the case of “aspectual templates”, where parameters form full models. As a first step, we show the consequent partitioning of modeling spaces into two subspaces: one dedicated to template modelers (“model for reuse”), the other one dedicated to application modelers (“model by reuse”). Respective activities are identified, but also back-and-forth between them. From this, engineering operators are isolated and formally specified. Finally, reusable technology within Eclipse is offered.