You can use standard functions on the entity level to define
- Calculated fields
- Entity filters
Calculated fields will not be stored in a data store but the system calculates their values. The values will be recalculated by the front end when changed by the user.
These functions are built-in and you cannot define your functions.
If you need special calculations to take place or if you want to persist the calculated values you need to use Data Structure Rule Sets.
If you need special functions when defining filters use Database Functions.