Every object by default contains no custom properties, but you can add them to an object to store values.
You can add or change a property using the property inspector window or through script code.
A custom property has the same behavior as regular 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 for programmers. The idea of linking the data with the object using it is productive. 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 allows it to be clear where the data is, and why.
You can see and edit custom properies with the property inspector: as well: