(a) At the option of the vessel owner, a vessel having a second deck is assigned either two gross and two net tonnages or one gross and one net tonnage.
(b) If two gross and two net tonnages are assigned, the higher tonnages (i.e. those based only on exemptions under Sec. 69.169 (a) through (d)) are applicable when the upper edge of the tonnage mark is submerged and the lower tonnages (i.e. those based only on all exemptions under Sec. 69.169) are applicable when the upper edge of the tonnage mark is not submerged.
(c) If only the low gross and low net tonnages, as calculated under paragraph (b) of this section, are assigned, these tonnages are applicable at all times. On these vessels, the tonnage mark must be located in accordance with Sec. 69.177(a)(6) at the level of the uppermost part of the load line grid.