The use of Settings models
Besides building pages, actions and your datamodel, records in the models can also be part of the structure of your app. Things about a set of data that would never change and be used in filters for variables and actions. For example countries, languages or categories. You want this data to be identical in every (sandbox) environment of your application. This is where Settings models are used for.
Enable Settings model
You can enable the settings model option in your app. You have to open the model in your Data Model and edit it. You will find a checkbox that says Is settings model. By checking this checkbox, the model will become a settings model and share its data between the main app and its sandboxes when merged.
After turning a model into a Settings model, records in the model will be carried over when merging the sandbox. This means it will overwrite any existing records, breaking associations with other records. The records from the sandbox are stored in the other environment, including id's.