The wording in 13.4 is very confusing:
range from HQ to unit for purposes of determining attrition, the hex path cannot be blocked by enemy units or opposing ZOI. Attrition is calculated and applied simultaneously, so it is possible that opposing units can mutually attrite each other.
From the first sentence it appears that a unit simply needs to be able to trace range through any hexes to a friendly HQ (that are not blocked hexsides). The 2nd sentence seems to indicate that unit can block supply...which is correct?
Can someone please explain this in more detail?
Using only part of the paragraph will certainly do that...
The two sentences above the ones you quoted concern unsupplied units that are within range of an HQ are not eliminated. The sentence used is defining how that range is calculated. Simply stated an OOS unit on its reduced side that is within the range, counted in hexes, unblockable by enemy units and ZOIs is not removed from play.