If you notice, within the app, your shop's fulfillment services automatically appear as sections/product groups within the app.
Products that belong to each fulfillment service are automatically assigned to the Default group within each fulfillment service section.
So, without doing any product assignment at all, products will already be sorted into their proper fulfillment service, based on the fulfillment service value you have already set for the product.
Default Fulfillment Service
By default, when you create a new product with Shopify it belongs to the 'Shopify' fulfillment service.
This Default fulfillment service is represented in the app by the My Products section at the top of the Product Groups tab.
Additional Fulfillment Services
When you create products from an app like Printful, or some other fulfillment provider, you'll notice that those products already show up in the 'Printful - Default' product group.
What happens when you assign a product belonging to the Printful fulfillment service to a product group within the My Products section?
Well, in this case the app will overwrite the product's fulfillment service value from Printful -> Shopify.
In most cases you do not want to change the product's fulfillment service.
Therefore you should instead create a product group within that fulfillment service's section.
If you notice, each fulfillment service section has its own '+ Add Product Group' button.
You should keep products belonging to a fulfillment service, within groups belonging to that fulfillment service within the app.
So if you want to make product groups, make a product group within that fulfillment service.
Only assign products from one fulfillment service's group to another if you feel like you totally understand what is going on with your fulfillment services and have a good reason to be re-assigning the products to a different fulfillment service.
If you don't have a good reason to be re-assigning products from one fulfillment service's group to another, then you should you first just try making a product group within the fulfillment service the product already belongs to.
If you do re-assign a product to a Product Group within a different fulfillment service, it will change that product's fulfillment service variable, and may break the connection of those products from being fulfilled properly.
So if you see the warning in the app when re-assigning, do not proceed with the assignment unless you are fully aware of the fact that product's fulfillment service will be modified/updated in the Shopify admin.
Feel free to contact us for more of any explanation if you need it!
If you aren't sure about it, just contact us, and we'll take a look and advise you further on whether you should re-assign the products or not.