Specifies that only top hits should be written to the
output files. A top hit has the best identity
(if the -id option is specified) or the best
expectation value (if -evalue is specified). If
two ore more hits have the best value, then they are all output.
Note that the default value of -maxaccepts
is 1 for commands based on the USEARCH
algorithm, which means that at most one hit will be found and the -top_hits_only
option therefore has no effect unless -maxaccepts is set to a value > 1.