As far as I know, the classes originated with the old-growth studies in
Great Smoky Mountains National Park in the 1990's. Rob Messick then
modified them slightly, and used them in his Old-Growth Forest Communities
in the Nantahala-Pisgah National Forest. The map uses the classifications
as defined in that literature; specifically:

Class A: "old-growth forests where no significant signs of human
disturbance to the forest canopy or understory could be determined.
Canopies are dominated by older trees generally over 150 years of age.
One hundred and fifty years is considered an appropriate coarse filter for
old-growth candidacy as this corresponds to a period when logging was
limited to areas near early settlement sites.
Note: Oak communities, where American chestnut was once most prolific,
have often changed dramatically due to the effects of American chestnut
blight. For this reason they often do not receive class A status..."

Class B+: "old-growth forests that have both class A and class B
characteristics. Sites in this class tend to be large, with numerous
forest communities, making it difficult to categorize the whole site.
Uncut forests with canopy trees at or above 150 years may be present in
these sites, yet the effects of disturbances such as blowdowns, American
chestnut blight, or fire may be present in other forest communities within
the site."

Class B: "old-growth forests exhibiting one of two different conditions:
1) the canopy is dominated by old-growth trees, yet signs of past human
disturbance to the forest canopy or understory were found (generally a
half century ago or longer). These stands have often been heavily
impacted by American chestnut blight. Culling may also have occurred.
2) no sign of past human disturbance could be confirmed, yet the forest
canopy is dominated by younger forest. These stands can range from 100 to
150 years in age and were possibly affected by natural disturbances."

Class C: "forests with obvious signs of past human disturbance, yet
containing appreciable old trees in the canopy or higher tree diversity
than surrounding forests. Forests in this class are suitable for
old-growth recovery. Some sites in this class are small. Others form
buffers for class A, B+, or B old-growth. Forest in this class usually
did not have extensive field work done in them due to time constraints."