Most building blocks are a GenericPropContainer, meaning that arbitrary key-value pairs can be stored in them, namely:
The view variants will reflect the generic properties of the handle variants.
A generic property key is always a String, and a value can be one of String, Real, int or bool. For each of these data types, methods to retrieve and store values are available both in Python and C++. For more details see the documentation of GenericPropContainer.