Custom Attributes are used by Compatible Units (CU).

Custom attributes allow the configuration formula to use values defined for Planned Quantities and Durations in Compatible Unit templates.

For e.g. You define the CU template for 1 sq. m of a tiled floor. Execution item connected to the CU template will include material and resources quantities required and duration (planned hours) needed to build it.

When you create the actual compatible unit from the template you give the exact floor area (a characteristic value) and the formula will calculate the total duration and material and resource quantities needed by applying the configuration formula with Duration and Planned quantities defined in relevant templates. (template defined for 1 sql m).  

Thank you very much.