The product and customer inputs let the user pick a set of existing products and customers from the master data. The two inputs work in the same way, just use different tables for data selection.
Product
The product filter allows the user to provide a set of products as an input. The end user can either select products from a list, or provide a filter that will be used to query the product master table.
Customer
The customer filter allows the user to provide a set of customers as an input. The end user can either select customers from a list, or provide a filter that will be used to query the customer master table.