Formula Editor

You might want to write a formula to calculate durations or capex, and you can do that anywhere in the Planner where you see a "fixed duration" or "calculated" drop down selection. Choose the calculated option to enable the formula editor below

When you start typing an attribute or function name, or if use the hotkey (Ctrl-Space) while editing a formula, you'll see a list of all matching attributes/functions with descriptions:

If you click on one of these or hit enter while its selected, then it will insert that attribute/function into the editor. 

The descriptions also stay active while you're editing the formula, so it's easy to reference in case you forget what a specific function does, or how the function is supposed to be used.

The formula editor will also automatically expand to multiple lines when necessary, so its much easier to write longer formulas:

See this help file for an example formula that references a lookup table