class_docstr: A class that stores variables for internal purposes in core during fit
  and predict. For instance, to manage dataflow properties.
code: def flow_mask(self) -> list:
      return self.features_mask.get("flow_lens")
  
file_path: FEDOT/fedot/core/data/supplementary_data.py
imports: from dataclasses import dataclass
  from typing import Dict, Optional
  import numpy
  from fedot.core.repository.operation_types_repository import OperationTypesRepository
  from fedot.core.repository.tasks import TaskTypesEnum
label_desc: Use image node_id 2 for calling the SupplementaryData obj's underlying member method code with example usage: obj.flow_mask() and returns: self