Thanks to the integration with the Nexar API (formerly Octopart) the app can give you quotes with real-time pricing from online suppliers such as Mouser, Digi-key, Farnell, etc. However, you can also add your own by creating a supplier, adding parts under it and as many (custom) quotes as needed.
These custom quotes can then be used to price BOMs and purchase lists.
Sales-only suppliers and the quotes listed under them are available only when quoting Sales Orders (i.e. not in purchase lists or when quoting a BOM).
When selling parts, you can define a selling price on parts themselves, but if you require a bit more control on pricing then you can create a sales-only supplier, which would represent your own business.
This is useful if you want to define prices in different currencies and based on volume purchasing (price breaks).
Preferred suppliers can be defined in
Settings - Workspace - Quotes. When automatically assigning quotes, the app will try to find quotes on these suppliers, in the same order as listed.
Don't forget to add your custom suppliers in the preferred suppliers list. This is not done automatically so you can define the order (i.e. priority) in which the app looks for quotes.