Hi @ndy ,

It depends on what have to include in your module. 

Since you named it *_BL, I would assume that you will only have entities and logic. In most cases, you need to choose Service as Service module type does not include any UI components.

In some cases, if it requires you to have Core Widgets (common reusable widgets which are UI components), then you have to choose either Reactive WebApp or Blank. Take note that Reactive WebApp will include some default UI components and Blank will be a clean slate without any additional components.

You can refer to the following links for more information regarding some of the module types:

Libraries: https://success.outsystems.com/Documentation/11/Developing_an_Application/Reuse_and_Refactor/Libraries

Service:
https://success.outsystems.com/Documentation/11/Developing_an_Application/Reuse_and_Refactor/Use_Services_to_Expose_Functionality#Service_Modules_and_Service_Applications

Extensions:
https://success.outsystems.com/Documentation/11/Extensibility_and_Integration/Extend_Logic_with_Your_Own_Code/Extensions

I would also suggest to go through the architecture training and documentations:

https://success.outsystems.com/documentation/best_practices/architecture/designing_the_architecture_of_your_outsystems_applications/

https://learn.outsystems.com/training/journeys/architecting-sustainable-apps-648

I hope this helps.

Regards,

Bryan