Door 6 is one of three choices in the third set of numbered doors that the players encountered, the other two being Door 1 and Door 2. These doors are not in the same room or on the same deck, unlike the previous doors.
Door 6 is on the E Deck of the ship. Junpei and June find it shortly after exiting the Saturn elevator on the right. Near the door is a burnt map of E Deck. Door 6 leads to the steam engine room and cargo room.
Door 6 was one of three doors the players could choose from in the third set of doors. It was located on the E Deck. The only way to get to Door 6 was by using the Saturn keycard in the elevator behind the staircase on the C Deck.
After going through all three doors and discovering Snake's fate, the remaining players decided to vote to decide who would take each door. Their choices were written on scraps of paper from Junpei's notebook, to make sure no one would change their vote for any reason. The results were:
Unknown to the others, Junpei had three papers. One was with the other papers while the other two were hidden, one in each hand. That way, he would be able to change his vote after reading everyone else's choices. However, he initially puts the paper with Door 6 written on it into the pot, so he doesn't have to switch his paper, covering up his plan.
If Junpei chose Door 6, he would go with Santa and June, with the addition of Ace.
1+3+5+6 = 15 = 1+5 = 6
This left Clover (4), Seven (7), and Lotus (8), who would go through Door 1, on Clover's insistence.
4+7+8 = 19 = 1+9 = 10 = 1+0 = 1
With that finished, the groups go to their respective doors.
Behind Door 6
The DEAD of Door 6 was close to the entrance and they all authenticate without trouble. Junpei momentarily reflected on his reaction to the beeping before they all explore the steam engine room.