Taxonomic groups in an unrooted tree(!) can be defined by
the branch at which the group is located
the direction into which the group points (i.e. which of the 2 subtrees that are connected by this branch does form the defined group)
In rooted trees the branch location on its own suffices as group-defining criteria, if we assume all groups point away from the root. This is the way taxonomic groups are handled in most software and so did arb (up to version 6.0.x).
This simple handling of groups was not able to reflect the semantic change which is implicitely applied by moving the trees root into a group.
To solve this problem, arb 6.1 introduced the concept of "keeled groups".
A keeled group may be interpreted in 2 (semantically equivalent) ways:
the keeled group "points upwards" (i.e. semantically it defines a taxonomic unit containing the two OTHER subtrees starting at its parent-node)
the keeled group "graphically embraces the inverse subset of species" compared with the original unkeeled group
To indicate keeled groups, their names are prefixed by exclamation marks, e.g. shown as '!group' which should be read as "not group"; this emphasizes the 2nd interpretation given above.
When the root of the tree is placed somewhere inside a group named 'Bacteria', that group gets keeled over. Its name now gets displayed as '!Bacteria' (should read as "not Bacteria") and the group does embrace the rest of the tree, i.e. "everything but Bacteria".