New with 6.1 is the concept of Asset Categories. An Asset Category is a simple tag on an asset that allows for several advanced features related to asset management, including hiding and displaying metadata. In 6.1, the feature is still quite barebone as it lays the foundation for future improvements to the overall asset model in KeyShot Dock.
...
Sealed: If a category is sealed, it cannot have any child categories.
Color: This is an optional color for the category. It is purely for display purposes and serves no functional purpose otherwise.
...
A metafield can be assigned to only be available for assets in specific categories. To do this, go into System => Metadata and find the field to configure. As part of the metafield's settings, an option for "Restrict to asset categories" is now available. The metafield will be available for all assets assigned to either the categories here or any child categories of those.
If a metafield is not restricted to any categories, it will not be available for any assets.
To make a metafield available to all categories, assign it to Root.
This same assignment works on Metafield metafield groups. Note that a metafield has to be visible from both the field itself and the group it is in. A warning will appear if you misconfigured this, but it will not prevent you from saving.