Not sure of the correct use IP2Location database

Question

I've used:-
$db = new \IP2Location\Database('./database/IP2LOCATION-LITE-DB3.IPV6.BIN', \IP2Location\Database::FILE_IO);
it seems to work OK.

IP2 docs. says "Class Database, Expects 2 input parameters"
  Full path of IP2Location BIN data file. -- OK
  File Open Mode. -- Why does it also need "\IP2Location\Database::" ?

Tried not using "\IP2Location\Database::FILE_IO" just using "FILE_IO".