What does it mean when they say they will "assign" your seat at the gate? My friend is traveling in February with her two girls (6 & 7) and on the last leg of the four flight segments their mom was assigned a seat but the two girls the Delta agent said would be assigned at the gate. Should they keep checking on-line to see if a seat opens up and grab it? The agent said they would have a seat but I'm just wondering how assigning at the gate works? Thanks everyone.