A single custom variable can have only a single value defined per business unit or project.

For example, the project has a "Sign off Project" form. The form contains a question "Total Allowed CO2 Emissions", with the default answer set to the custom variable called "CO2 Allowance". If the value of the custom variable has not been specified for the project, the business unit's default value for the custom variable is displayed as the answer to the question on the form.