I believe I have solved this issue myself by adjusting the product item info!
When I want to put a product on sale for eg promotion purpose of 10% discount sale (but do not want to add this to the already discounted shopper group discount):
1/ I created a product category eg called '10% discount'
2/ I created a calculation rule of 10% discount to be applied to the '10% discount' product category
3/ I put the product to be promoted under its normal product category and the 10% discount product category
4/
assign the last (from back-end product page) to the shopper group of '-anonymous-' and '-default-' only - somehow this excludes all other shopper groups from the 10% discount sale (include 'retailer' group in my case).
I hope I have made above understandable.
regards
En Jie