Whats the value behind $annual_revenue

Hey community :-) I would like to display the annual revenue in the accounts module.

Expectation: use $annual_revenue in a calculated field to get a rollup sum of opportunities closed won in the appropriate timerange

Instead: It's always 0, tested different field formats (text, float, currency, with and without number and string formula.

I know we kind of mistreated the revenue fields in the opportunities-module, so I'm guessing $annual_revenue is adressing data that is not populated at all.

So please help, what's the magic behind the annual revenue?