The Latin is legatus pro praetore, which is to say that Labienus was a legate with a separate propraetorian imperium of his own. Labienus was not just a legate deriving his command from Caesar's proconsular imperium, but with propraetorian imperium (i.e. Standing in for a praetor) Labienus held separate imperium of his own. As such he was entitled to six lictors, could make certain judgments on provincials, command of a praetorian army (I forget how many legions that is), etc.