I don't think I understand
You only have to update the add to cart buttons to add content to those columns. I don't see how having placeholders would help... if you add a column you will need to update the add to cart server behavior to specify the contents. 
I don't think I understand your response.
Perhaps restating will confirm my intention...or not...
In order to do the discount on category, I need to store the category id somewhere/product. I could do a plain old session variable but it wouldn't be tied to each row product added.
So, when I open the ecart window, I see the "General, Columns, Calculations, Discounts, Charges, Taxes" tabs. When I go to "Columns" I see a list of items like "Price, Name, Id, weight..." that are added/line when a product is added. I need to add one more "Column" here called "Category" that will record the category of each product and only those products will get a discount of 25%.
My past experience tells me that if you change the Columns by adding one more, and if you post the "WA_eCart_Definition_VB.asp" and "name_eCart_VB.asp" that get updated when you change anything in the ecart, the site dies and you get a major error.
The only way I know to do this is to go into each page and update the (as you said) the add to cart page and I'm wondering if this is the only way. I have a lot of pages and I want to avoid this.
Is this clear?
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By the way, what would the rule look like?
I know it would be something like the following but what would it look like if I ONLY want it to apply to items that are in that category and not in others?
25% off if category is (categoryID)
