I was wondering the same thing, found your question, then pulled up the metadata:
The NLCD 2016 Land Cover Change Index is a raster image with an
accompanying look-up table that describes the land cover change
represented by each index. Previous releases of NLCD land cover
covered one target year, and a land cover change index was compiled of
change from every class in the previous target year to every class in
the current one – for example from 2006 to 2011 or from 2001 to 2006.
NLCD 2016 Land Cover spans not one but seven target years: 2001, 2003,
2006, 2008, 2011, 2013, and 2016. Rather than compiling six land cover
change index products (one for each pair of adjacent target years),
there is a single NLCD 2016 Land Cover Change Index product whose
pixel values indicate land cover changes that occurred at least once
between 2001 and 2016. Each assigned image value represents either “No
Change” (Index number 1) or one of the changes described below.
Index number 2 identifies pixels that have changed from any land cover
class to Water, or from Water to any other land cover class, between
any two adjacent target years. Indices 5, 7, 8, 10, and 21 are similar
identifiers of change from or to a single land cover class.
Index 3 identifies changes from any land cover class to any of the
four Urban land cover classes, or from any of those four classes to
any other land cover class, between any two adjacent target years.
Index 11 similarly identifies changes from or to any of the three
Index 4 narrowly identifies changes only between Herbaceous Wetland
and Woody Wetland. Similarly, Index 6 identifies changes only between
Cultivated Crops and Hay / Pasture.
Index 9 meets a special requirement of shrubland research to
discriminate where change occurs on persistent shrublands verses
change on transitional shrubland areas such as regenerating forests.
Pixel /Index Value Description 1 No Change 2 Change from or to Water
3 Change from or to any of the four Urban classes (open space; low,
medium, and high intensity) 4 Change from Herbaceous Wetland to Woody
Wetland, or vice versa 5 Change from or to Herbaceous Wetland 6 Change
from Cultivated Crops to Hay / Pasture, or vice versa 7 Change from or
to Cultivated Crops 8 Change from or to Hay / Pasture 9 Persistent
Grassland and Shrubland change. This change index attempts to identify
changes to persistent Grassland and Shrubland areas, and to separate
them from transitional shrubland areas such as regenerating forests.
10 Change from or to Barren 11 Change from or to any of the three
Forest classes (Evergreen, Deciduous, and Mixed) 12 Change from or to
Identifying the specific changes from one target year’s land cover
class to another, and the specific target years when the changes
occurred, both require analysis of the land cover data itself.