See 2.2.52.1.8 of the RID:

"Classification of organic peroxides not listed in 2.2.52.4, 4.1.4.2 packing instruction IBC520 or 4.2.5.2, portable tank instruction T23, and assignment to a collective entry shall be made by the competent authority of the country of origin. The statement of approval shall contain the classification and the relevant conditions of carriage. [...]"

This BAM No. represents the collective entry itself -  has been assigned packing method OP6 according to 4.1.7.1.4 b) of the RID. A packing method with a lower OP number may be indicated in the approval certificate of the competent authority if the organic peroxide meets the criteria of the Manual of Tests and Criteria, para. 20.4.3 c) only in a smaller packaging.