The architectural rationale is the set of reasons behind the decisions made when designing the architecture of a software system. Normally, this rationale remains in the minds of designers and others involved in the design. Therefore, the reasoning behind the decisions that underpin the architecture model may be lost if not properly documented, causing maintainability problems in the software. In practice, the rationale is not documented or is documented in the middle of the architectural descriptions, which makes it difficult to understand and support subsequent decisions, within the development and maintenance of software, which is more critical in the agile approach to development. To address this problem, this paper proposes a documentation approach that combines rationale modeling with a focus on decisions made in projects that use agile methods, to specify a language that provides the basis for the construction of a tool that we have called DRML (Decisions and Rationale Modeling Language). The language is evaluated to document the rationale in the framework of the Unique Indigenous Information System (SUIIN, for its acronym in Spanish) project, in the context of a public entity that within its processes has a work team made up of systems engineers focused on software development. This evaluation has established that the approach provides sufficient expressiveness to document the decisions and their rationale, however, it has limitations to scale the modeling for a large number of decisions and their relationships.