All species have a flag which can be set to FALSE or TRUE. All species with the flag set to TRUE are called marked species. Normally this flag is used to specify a subgroup of species for editors, tree building programs ...
Unmark all species: <ARB/Species/Mark species/Unmark all Specie>
<ARB/Species/Mark species/Species in Tree/Mark species in Tree>
or
Select the <MARK MODE> 2a. Press the left mouse button somewhere outside the tree to mark the whole tree. 2b. Press the left mouse button at a subtree to mark it.