Fixed level of detail expressions aggregate at the level of detail that's specified by the list of dimensions in the code, regardless of what dimensions are in the view. For example, the following code returns the average Rent per Building, regardless of what other dimensions are in the view:

{FIXED [Building] : AVG([Rent])} 

The following two snippets of code represent a fixed calculation of the average rent for the entire data source (or the subset defined by a context filter):

{FIXED : AVG([Rent])} 

This is the other code snippet you can use:

{AVG([Rent])}