Field help (SCSFLG) 

A Service type category with a Sold flow is a category relating to subcontract products.
A product of this category can be used in:

If this particular flow type is Sold, then the product is always Deliverable (so it can be received).

However, with a Sold flow type only, the product does not always have to be Deliverable. In this case, the product is a sheer service product, which is not included in deliveries.
This product will be invoiced through invoicing open items.