code: def _Bound_TP(T, P):
      
      region = None
      if 1073.15 < T <= 2273.15 and Pmin <= P <= 50:
          region = 5
      elif Pmin <= P <= Ps_623:
          Tsat = _TSat_P(P)
          if 273.15 <= T <= Tsat:
              region = 1
          elif Tsat < T <= 1073.15:
              region = 2
      elif Ps_623 < P <= 100:
          T_b23 = _t_P(P)
          if 273.15 <= T <= 623.15:
              region = 1
          elif 623.15 < T < T_b23:
              region = 3
          elif T_b23 <= T <= 1073.15:
              region = 2
      return region
docs: Region definition for input T and P
  
      Parameters
      ----------
      T : float
          Temperature, [K]
      P : float
          Pressure, [MPa]
  
      Returns
      -------
      region : float
          IAPWS-97 region code
  
      References
      ----------
      Wagner, W; Kretzschmar, H-J: International Steam Tables: Properties of
      Water and Steam Based on the Industrial Formulation IAPWS-IF97; Springer,
      2008; doi: 10.1007/978-3-540-74234-0. Fig. 2.3