You can apply price list to sale in two different ways:
Assign price list to cash registers
When price list is assigned to cash register, it becomes default price list for all sales.
To assign price list as default price list of the cash register, go to Settings > Order and payment and select available price list from the list.
Once you save settings, selected price list will become default price list. To revert to default product prices, select "(none)" in the list.
Assigning price list to cahs register in Management section
If you are using Aronium Pro, you can assign price list to cash register directly in Management > Cash registers section. With Price lists plugin installed, edit cash register will have an additional option to select price lists to be used on selected cash register.
Assign price list to a customer
Alternatively, you can assign a price list to a customer. Once price list is assigned to the customer, every time you select that customer, prices from assigned price list will be applied to the current sale.
To assign price list to a customer, go to Management > Customers & suppliers, select a customer and open it for edit. From there, go to Payment terms tab and assign a price list.