The Terms of Reference (ToR) form the framework given to the visitation committee to assess the research. The ToR are set by the boards of the institution to which the research unit belongs. All being well, this should be done in consultation with the boards one level below, such as faculty boards and institute managers.
If the SEP is followed, it should not be possible for a board to draw up a single frame of reference for all the research in its institution (see p. 30 SEP). It is extremely important that the ToR not only reflect the scope of the assessment (e.g., national assessment or stand-alone assessment) but also the nature of the domain. This is particularly applicable to most domains in the humanities, where different criteria are often used to determine quality and impact than in the natural sciences and large parts of the social sciences. This has implications for the selection of indicators, for example.
It is essential that the ToR frameworks are properly attuned/coordinated before the self-evaluations are written.