Product stocked at both standard and app fulfillment service location

Good morning,

Is it possible to have a product (configured to be stocked in Multiple locations) to be stocked in a mix of “standard” Shopify locations as well as an app-fulfillment-service location at the same time?
On one of our customer’s shops, when we open the product’s Edit locations window, there’s a list of standard locations as well as the app-fulfillment-service location (highlighted with an app tag next to its name).
We struggle to build a similar product configuration - what are the necessary circumstances for an app-fulfillment-service location to show up among the standard locations?

Thank you!
Piotr