Accounting Unit is the third subdivision of HUC11. Cataloging Unit is the fourth subdivision of HUC11. In cases where the units digit is not zero, the units digit supercedes the tens digit. If there are level 3 tributaries that enter the level 2, the first is given the next stream number. If there are any level 4 tributaries that enter the level 3, the first is given the next stream number.
Tributaries converge to produce higher order.
Linear section of streams. Region is the first subdivision of HUC11. Unique refers to one specific line segment or stream reach.