A promotion/sales is a general discount you apply to products you sell on your website
If discounts can not be combined the priority will decide which discount is applied (lowest priority number first e.g. 1 then 2 etc.)
Cart total, ie the discount type (fixed amount, free shipping or percentage) is applied to the total cart amount. You can set the cart total to be a minimum or maximum value.
An online coupon refers to a discount code or number that can be redeemed on a website. Like traditional coupons, online coupons are used to attract new customers and increase the loyalty of new customers.