What type of expressions cannot be deleted?

System expressions are integral to the functioning of the software and represent the fundamental parameters and settings that govern the overall behavior of the application. These expressions are built into the software's environment, and as such, they provide essential references for various operations and functions within Siemens NX.

Because they define critical aspects of the system’s functionality, deleting system expressions could lead to instability or disrupt key processes and calculations. Therefore, the inability to delete them ensures that the integrity and operational stability of the environment are maintained. The other types of expressions — feature expressions, user-defined expressions, and unused expressions — are typically more flexible and can be adjusted or removed depending on the user's needs or preferences. This distinction emphasizes the critical role that system expressions play within the software ecosystem.