A higher suitability in one of these dimensions might not correspond to a high level of suitability in the other dimensions. Given preference to the different criteria will depend on the interactions among them; we then introduce didactical suitability as a systemic criteria of adequacy and appropriateness regarding the global educational project. This didactical suitability is relative to temporal, contextual and changing circumstances, which requires an inquiring and reflective
attitude from the teacher and the people sharing the responsibility of an educational project. The theoretical tools developed in the onto-semiotic approach to mathematical knowledge and instruction can be applied to analyse the teaching and learning process implemented in a particular teaching session, the planning and development of a didactical unit, or at a more global level, in the design and implementation of a course or curricular proposal. They also can be useful to analyse partial aspects of a study process, such as the didactical resources, textbooks, students’ answers
to specific tasks, etc.