For 12.2, please use the following note:
Creating a Custom Application in Oracle E-Business Suite Release 12.2 (Doc ID 1577707.1)
You can also review:
All the documents are reviewed and there is no mention of how to use or ignore of parameter base_product_top in EBS12.2. But according to ebs 12.1, we should be able to use this parameter to direct custom apps to desired location. Best the way is to try to add your own custom app to one of your test instance. You can always de-register it later on. That way you will know what I am talking about.