The main pdf documentation "MRN Data Models and Elektron Implementation Guide" does not mention this field at all. There is a discussion about the field in the N2_UBMS comparison page which is missing from the pdf documentation:
In MRN, the classification is defined in following JSON fields.
- messageType field, an integer field that use the same enum value as FID 3 of N2_UBMS.
- urgency field, where 1 is an alert and 3 is the newsbreak story article. The number of take is then defiend in takeSequence field.