A calculated field is a column that holds values that are not entered 
or changed directly but are instead calculated automatically from other 
values in that row of from rows in other tables related by a key field. 

There are two types of calculated field values:

*Not stored*: calculated fields that are not stored in the database are 
recalculated every time the row is accessed. They are defined by a 
JavaScript expression where the values of the other fields are in scope.
They are suitable for quick calculations that depend only on other values 
in the same row. Because they are recalculated they are guaranteed to be 
up to date. Non-stored fields cannot access data in related rows by join 
fields or aggregations and cannot use asynchronous functions provided by
modules. They can use synchronous functions.

*Stored*: the values of stored fields are stored in the database table. 
They are more flexible in the way they can be defined. Stored fields can 
be defined either by a JavaScript expression or by an aggregation. When 
defined by a JavaScript expression, join fields can be accessed by the 
dot notation. Stored fields may also use both synchronous and asynchronous f
unctions provided by modules. Asynchronous functions can in principle do 
input/output including calling APIs. This expression will be evaluated 
every time the row changes.

Stored fields that are defined by an aggregation are automatically updated 
when one of the child rows is updated. Stored fields that are based on 
JavaScript expressions involving join felds are not automatically updated. 
You need to run the recalculate stored fields action to update them. 