You can use standard functions on the entity level to define
Calculated fields will not be stored in a data store but their values are calculated by the system. The values will be recalculated by the front-end when changed by the user.
These functions are built-in and you cannot define your own 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, you need to use Database Functions.