It is possible to add custom attributes to objects (meshes, curves, lights and cameras).
Adding a custom attribute is achieved by selecting the attribute type in the 'Add' combo-box in the
parameter area. The pen icon after the attribute name opens the attribute configuration:
There, the user can define the name, the minimum, maximum and default values.
A custom attribute cannot do something by itself, but it useful when linked to another attribute
(of the same or another object) to control multiple attributes at the same time, or to regroup
attributes in the same place.