In which cases should data transfer be performed between different models or databases?

Data transfer between different models or databases is usually required in the following cases:

• Data migration during application updates: when the data model structure changes, for example, new fields are added or storage logic is modified, it is necessary to transfer old data to the new model.
• Switching to another DBMS or storage technology: if the application switches from one type of database to another (e.g., from SQLite to Core Data or Realm), existing data needs to be migrated.
• Combining data from different sources: when integrating multiple applications or services where data is stored in different databases.
• Performance optimization: splitting data into different models or databases to reduce load and speed up access.

In iOS, for example, when using Core Data, data migration occurs through migration policies that allow automatic or manual transfer of data between model versions.