Model-based engineering (MBE) contributes a lot to software reuse by abstracting technology independent models, their capitalization and then their reuse to pro- duce systems, ensuring early composition and checking capabilities. One step further, it was recognized that models could be parameterized in order to capture re- current modeling patterns. This led to the concept of "model template" as offered in UML. In this paper we con- centrate 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.