See 2.4.2.3.2.4 of IMDG Code:

"Classification of self-reactive substances not listed in 2.4.2.3.2.3, packing instruction IBC520 or portable tank instruction T23 and assignment to a generic entry shall be made by the competent authority of the country of origin on the basis of a test report. Principles applying to the classification of such substances are provided in 2.4.2.3.3. The applicable classification procedures, test methods and criteria, and an example of a suitable test report, are given in the Manual of Tests and Criteria, Part II. The statement of approval shall contain the classification and the relevant transport conditions.[...]"

This BAM No. represents the collective entry itself - it has been assigned packaging method OP6 according to 4.1.7.1.4.2 of the IMDG Code. A packing method with a lower OP number may be indicated in the statement of approval of the competent authority if the self-reactive substance meets the criteria of 2.4.2.3.3.2.3 only in a smaller packaging.