Expression properties are special properties that can handle logic. The value of these properties are not set by users like most properties, but can change depending on how the expression is evaluated. Within Betty Blocks there are many types of expression properties, these types determine how the output will be formatted.
Variables are key when setting up dynamic applications. Instead of entering set values everywhere, you want to use variables. Variables can be seen as placeholders for temporary values, used in the calculation of a certain value (functions and/or formulas).