overflow sump HELP

[cathyg_99]

so i plumbed it for the return and the larger 2" overflow i just figured the 1" bulkhead was so the tank could be drained completely.... till i put my finger in the hole and realized theres no cap/or threading....

so stupid question...... whats it for??
should i just put a 1" tube through it as a double overflow?

the sump is set up so it goes... from tank - skimmer - bubble trap- mechanical- live rock - return pump-tank