In order for an organization to be officially appraised for CMMI, and (if required) have their rating published, an external certified team of appraisers must conduct a review of the objective evidence that would support the CMMI Practices and Goals required for the given certification (rating). Typically this is a major undertaking with a small team of external auditors collecting information (via interviews and reading documentation etc.) then analyzing this information to produce an appropriate rating.
In addition to the actual rating the appraisal team will make recommendations on which process areas have deficient evidence that support the presence of a given Practice or Goal. The team would also advise the organization on how to make up such deficiencies for future appraisals. One major benefit of such an extensive external appraisal is to give the organization an insight into the strengths and weaknesses of their current processes.