IBA stands for Instance Based Attributes. As its name indicates, they are Instance based.
By mean of Instance based - they are defined on particular object type (WTPart, WTDocument, etc). For example if you look at Name, Number, they are available for all object types. We call them as Modelled objects.
Instead, IBA can be defined on either WTPart, WTDocument, etc.. they are specific for those types. Additionally, you can also define IBA for soft types too. It means you can define attribute say IBA_1 specific to soft type to softtype_1.