First of all, I would like to point out that Shopping cart price rule different from all others.
In Magento, the lowest amount of below prices will be used as the cart price whenever a product is cart.
- Catalog Price Rule.
- Group Price of product
- Tier Price
- Special Price.
Suppose, product prices are like
- Catalog Price Rule. is 200
- Group Price of product is 150
- Tier Price is 175
- Special Price. is 210
In this case, Magento will use Group price of product since which is the lowest amount of all prices as per above example. Hence the cart price will be 150.
Shopping Cart Price Rule is applied to the price that you have already in a shopping cart. Hence Price Rules has no direct relationship with above specified price items.