what are complete setups for sales order management module to use project

Summary:

I reviewed the official guide

Implementing Manufacturing and Supply Chain Materials Management

It directs to order management implementation guide

Set Up Projects in Order Management

But in the order management implementation guide, the content is not complete, and it is confusing

why do we need to create a new lookup type? how can a sales order specialist use this lookup type, sales order line type is a predefined lookup type

How to prevent order management sending order line to receivable? in the order implementation guide, there is no such content.

I cannot find the the Process Sales Orders for Projects opt-in feature