Every object contains no custom properties, you can add to it to store values.
You can add or change a property using the property inspector window or using script code.
A custom property has the same behavior of a properties, to create one just use set:
set mycustprop of me to "Hello world"
Custom properties are fundamental in LiveCode and change the way of coding programmers. The idea of linking the data with the object using it is genial. If you don't need it anymore for an object, you can delete it and also all its custom properties. This way the software doesn't continue to store unnecessary data. It becomes clear where the data are and why.
You can see and edit custom properies also with the property inspector: