What divides Hydrological Unit Code areas of the same level? I'm trying to better understand the Hydrological Unit Code (HUC) system of the United States. I understand the basic hierarchy of it (HUC4, 6, 8, 12) and watersheds/drainage areas. Here's what I want to clarify:

A group of HUC8s can be within a single HUC6, because each HUC8 ultimately drains to the pour point of their parent HUC6. What separates one HUC8 from another HUC8 downstream of it, both within the same parent Hydrologic Unit? I suspect the answer isn't the natural drainage area, because the downstream HUC8 would contain the upstream HUC8 in its natural drainage area - that's based on me thinking that as you shift the discharge point you're examining further downstream, the drainage area you're examining increases.

The most concise, relevant explanation I've come across is from the USEPA's EnviroAtlas Data Fact Sheet on the subject:


  A
  watershed  is 
  defined  as  the
  geographic  a
  rea  within  the 
  boundary
  of   a   drainage   divide.   Watershed   boundaries 
  follow  the  highest  ridgeline  around  the  stream 
  drainage 
  area; 
  the  bottom 
  of  the  watershed  or  the  pour  point  is  the
  lowest point of the land 
  area 
  where water flows out of the 
  watershed.
  Hydrologic   unit   boundaries   do   not   always 
  surround    a    complete    watershed    but    may    delineate 
  truncated portions of a larger watershed
  —
  for example, the 
  mid-stem   of   a   larger
  stream   or   river   along   with   the 
  tributaries
  in  that  area. 
  Hydrologic  units  are 
  generally 
  synonymous   with   watersheds   when   their   boundaries 
  include  all  the  source  area  contributing  surface  water  to
  a 
  single   defined   outlet   point.   This   distinction   between 
  watersheds and HUCs is important in the context of water 
  resources   data   analysis   and   water   quality   monitoring, 
  because  the  area  contributing  to  the  downstream  outlet 
  point in a single HUC may extend beyond its boundaries in 
  an  upstream  direction  to  include  a  number  of  other  sub-basin HUCs.


This still does not clarify exactly what separates one HUCx from another HUCx downstream of it, both within the same parent HUC?
I'm confused about your misunderstanding as the answer to your question is in the Wikipedia link you posted:


  Regions receive a two-digit code. The following levels are designated
  by the addition of another two digits.[8] The hierarchy was designed
  and the units subdivided so that almost all the subbasins (formerly
  called cataloging units) are larger than 700 square miles (1,800 km2).
  Larger closed basins were subdivided until their subunits were less
  than 700 square miles.[7] The 10-digit watersheds were delineated to
  be between 40,000 and 250,000 acres in size, and the 12-digit
  subwatersheds between 10,000 and 40,000 acres.[6] In addition to the
  hydrologic unit codes, each hydrologic unit was assigned a name
  corresponding to the unit's principal hydrologic feature or to a
  cultural or political feature within the unit.[7]


So, once you are outside of the 2 digit regions, HUC boundaries (or dividing/outlet points along a continuous stream) that separate an upstream watershed from one a downstream one (that should incorporate the upstream HUC using the classic definition of a watershed) are set to evenly divide up the larger (or containing) HUC.