You can configure discounts in 'Settings/General settings/Discounts'.
To name global discounts in sales, you can create a specific product (in the 'Products' tab).
In the discount settings, enter the ID of this product.
Global discounts will be assigned to this product.
Discount buttons enable to apply a discount with a single click:
The discount label is the name of the discount button on your sales interface.
Discounts can be in percentage or in value.
The value is either a rate (-10 for -10%) or an amount (-5 for -5 EUR).
The discount type (% or value) must be selected in the last drop-down menu.
You can hide or delete an existing discount.
You can apply line-item discounts (select the product in the sale and apply the discount) or sale wide discounts (click on the discount button below product keys in the sale). More information.