46 CFR §69.175

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