Calculation Variables can represent any other type of variable. Their value is interpreted when they are bound to an object. You can use basic math in calculation variables (like 3 + 5) and also modify text (like "text" + " here"). You can also use the values of other (non-calculation) variables by typing their name (if your variable's name has a space, use this["variable name"] instead). You can access the current resolution with resolution (which means you shouldn't name variables "resolution" or they won't be accessible from calculation variables).
To represent boolean values, you can use true and false. For text values, use "some text" and you can combine text with +. For colors, use [red, green, blue, alpha] where red, green and blue are numbers from 0 to 255 and alpha is a number from 0 to 100. All other values are just numbers (you do not need to write % for percents or ° for angles in calculations).
Calculation variables are calculated using JavaScript, so if you want more advanced functionality you can also use JavaScript directly in them.