Based on the Gross margin that is set on the settings page, the size of the 'C-category' is determined. This category will contain the products which will be the least profitable on an annual basis (longtail).
To quickly free up cash in this category, you can tweak a few settings in Optiply that should help with this. These are listed below:
Reducing the delivery reliability of the 'C-category' so that you will hold less safety stock.
Set the delivery reliability to 0% which will result in not having any stock of the 'C-category'. This way these products will only be sold on backorders and if they made enough profit they will automatically be moved up to the 'B-category'.
Look in the product details page and filter on the 'C-category'. Here you can check manually if your products are high in stock, if this is the case you can phase out these products so you don't keep them in stock any longer.