This Simply Catholic article Catholic Burial and Cremation explains the rationale behind the prohibition of spreading the ashes from a cremated person, along with the precondition that the cremated person needs to believe in the resurrection of the body. Instead, the ashes are to be buried or stored in a sacred place to encourage prayer on behalf of the deceased as a member of the communion of saints. The basis for the article is the 2016 instruction from the Vatican's CDF.
My question has to do with the authority structure and the consequent variations of local implementation of this teaching:
how much of the rules is set in canon law, how much by CDF, how much is set by a country's congregation of bishops (like USCCB)
whether bishops or even priests can relax or add further restrictions in connection with preserving the ashes
how much of the actual practice varies by country / diocese