When are expressions evaluated in a report? What is the default value before the items are rendered?

Hello,

I would like to know if expressions in a report are evaluated altogether when the report is launched, or only when the items are rendered (e.g. if I put an expression at the very end of my report, will it evaluate it after it has rendered all of it, or right away?). If the latter is the case, what is the default value before expressions are evaluated?

For example, let's say I want to check if something is true or false on the last row of a tablix. Will it instantly evaluate to true/false or not? What is the default value?

Is there a manual which references this?

Thanks for your help.