This symbol is a type alias.
Source:
MaterializableStructure = Union[
numpy.generic,
numpy.ndarray,
Iterable[Union[numpy.generic, numpy.ndarray]],
ForwardRef('MaterializableValueReference'),
Sequence['Structure[T]'],
Mapping[str, 'Structure[T]'],
pytype_extensions.Attrs[ForwardRef('Structure[T]')]
]