All products belong to 1 or more categories ...
When there are more than 1 category, then they are Linked Products ...
All products, regardless of belonging to 1 or more catgories, have a master_categories_id that is used for pricing purposes ...
Sale Maker peeks at this master_categories_id to know if a product is or isn't on sale ...
If you do not have this configured that when a product belongs to multiple categories then there is no way to apply a Sale to the Category of products as "sometimes" it would be on sale and sometimes not ... based on where it shows up ...
So, the Master Category ... the master_categories_id is the Category that "Owns" or "Controls" or "Rules" over the Product to manage pricing ...
It is also used to compute the products_price_sorter ... this is the bottom line lowest price you can get a product for ...
So, if a Product was in the Widgets Category and you decide to either add it to the Clearance category as a Linked Product or Move it from the Widget Category to the Clearance category ... the master_categories_id needs to be the Clearance category not the Widget category ...