To summarize B10.1, non-inherent terrain creates a crest line; inherent terrain honors the existing crest line.
The outline of non-inherent terrain, whether it be brush, buildings, woods, grain or whatever, is a crest line where it touches lower-level depiction and the ground under it is at the upper level.
Inherent terrain--e.g. orchard, crag, vehicle/wreck, bamboo, dense jungle, rubble--rises from the level of the ground terrain depicted in the hex, with it rising from the upper level where upper level ground is depicted and from the lower level otherwise.
Some maps have non-inherent terrain that have crest lines drawn within them, and these crest lines can be optionally followed instead of following B10.1 strictly. As far as I know this only applies to grain, and only on some late boards such as 59H3. You may also attempt to guess the crest lines in woods hexes that have peek-a-boo holes to show the hex height such as 47G3 or 47K3. Depending on the hex and the artwork, that can be a lot trickier. Using B10.1 without modification, all the non-inherent terrain that surrounds such a hole is at the upper level to the hex edge.
JR