TreeSelect
TreeSelect[tree,crit] picks out all subtrees treei of tree for which crit[treei] is True.
TreeSelect[tree,crit,n] picks out the first n subtrees for which crit[treei] is True.
TreeSelect[tree,crit,levelspec,n] picks out subtrees on levels specified by levelspec.
TreeSelect[crit] represents an operator form of TreeSelect that can be applied to a tree.
Please visit the official Wolfram Language Reference for more details and examples on core symbols.