What type of expressions cannot be deleted?

System expressions are predefined within the software and are integral to its functionality. These expressions typically represent fundamental operations or frequently used functions that help maintain the integrity of the application. They cannot be deleted because they serve essential roles in automating processes, ensuring consistency in calculations, or providing critical system-related data.

On the other hand, user-defined expressions, conditional expressions, and mathematical expressions are created or utilized by the user for specific tasks and can be altered or removed as needed. This flexibility allows users to customize their workflows and models according to their individual requirements. Thus, only system expressions are protected from deletion to ensure the software operates effectively without losing essential functionality.