Sales Managers and Sales Users are extremely similar, and are full users of the incentX suite. They can access all parts of the incentX system, and can receive payment via commission contracts. Your employees who are responsible for generating sales for the company, and are compensated by their performance, should be Sales Managers/Sales Users.
CRM Users are a limited user type, and they cannot receive commission payments. This user type is intended for front-office workers who help existing customers place orders, but do not generate new sales for the company.
External Referral Partners are not really users of incentX - they are for sales people who do not work directly for your company, but are paid commission for selling your products. They cannot access incentX, but they can receive commission contracts.