Reason is explained here: https://community.jboss.org/thread/212115

Here's the JavaDoc showing the use cases for this enum type.

public enum AvailabilityType {

    /**
     * Resource was externally removed.
     * Example situations:
     * <ul>
     * <li>For a server resource, the agent observes the server is no longer installed.</li>
     * <li>For a .war deployment, the .war is no longer running on the server</li>
     * <li>For a file, the agent observes the file is no longer on the file system.</li>
     * <li>For a database table, the agent sees the table was removed.</li>
     * </ul>
     * The difference between REMOVED and DOWN is that REMOVED resources were
     * likely intentionally removed and advises administrators that this
     * resource should no longer be inventoried.
     * <p/>
     * This availability should only be used if the agent is certain the
     * resource was removed. These are examples where DOWN would be the correct
     * type:
     * <ul>
     * <li>For a database table, the agent has no permissions to access the
     * parent schema. It is possible the table still exists.</li>
     * <li>For a file, the agent cannot access the parent's directory.</li>
     * <li>For a .war file, the server was shut down as part of upgrade.</li>
     * </ul>
     * As a rule of thumb, if the parent of a resource is available,
     * and if at least one 'siblings' of a resource are available,
     * but this particular resource is absent, then it is suggested to
     * return this availability type.
     */
    REMOVED
    ;