There are two ways objects, definitions and effects differ from proplists:
- Functions can declare that they want one as a parameter. Engine-defined functions get a pointer to the internal C++ type instead of a plain pointer to a proplist.
- Some properties behave differently. For example, foo_effect.Name=42 throws an error, where foo_proplist.Name=42 just puts 42 into the Name property.

For other kinds of data, like the global variables or the actmap, the engine uses ordinary proplists and merely reads and writes the properties.

The reason you can call engine-defined functions on objects is that every object has a definition as a prototype, every definition has the Global proplist as a prototype, and the engine puts the functions into properties of the Global proplist.

It'd be cleaner to split the Global proplist into one with just the object functions and one with the global functions, and make the former the prototype of the definitions and the latter the source of global functions.

For new engine functions that aren't intended to be called on objects we might want to consider putting them into a new proplist, and making that the prototype of the proplists the script works on.