Tables Vol. II: Missing Man Table

Photo Credit: Spc. Elisebet Freeburg, Joint Sustainment Command-Afghanistan, PAO

At official military and naval ceremonies in which dining takes place, US tradition states that a single,  circular white-cloth table is set for one, symbolizing the absences of soldiers who are prisoners of war or missing in action. The table might be set for one to symbolize the vulnerability of one isolated prisoner, or for larger events set for six, to represent the members of the five armed services (Army, Navy, Marine Corps, Air Force, and Coast Guard) and a sixth place setting for the civilians who died during service alongside the armed forces or who are missing during armed conflict. The table is round to represent the everlasting concern of the survivors for those loved ones. The chairs always remain empty to remind all of those who are not present. A slice of lemon reminds everyone of their bitter fate. The salt symbolizes the families tears shed as they wait for the return of their loved ones. The wine glass is inverted because the missing and fallen cannot toast. A yellow ribbon is tied to a vase with a single red rose to remind everyone of the families and loved one in harm's way.
You can read more on Wikipedia.