You want to sublet your apartment for several months at a time (e.g. to a person studying or working etc. in Berlin during this period).
You can usually do so without a permit from the Bezirksamt (local authority) and without a registration number if your guest will be living in the apartment for at least 3 months. It is important that your guest uses your apartment to live in (e.g. during an international assignment or university studies etc.) and not just for holidays, and you do not offer hotel services.