What are the possible values for pricing rule?

When a product is priced it's being done via pricing rule, the possible values are: 

• bulk_pricing::#{gid} - products in bulk pricing and the id of it's group. 
• campaign::#{campaign['id']} - products in promotion and the id of the promotion. 
• external - When a product get external recommendation (from API).
• manual - if the product recommendation price was set manually by a user. 
• -1 - It means that a recommendation is Unavailable. 
• rule number - A rule number got from the strategy the product belongs to.