The episode is bookended by the concept of passing through to "the other side," specifically through the story of the veteran Don meets at the bar.
We see Peggy fully settled into her role at Cutler, Gleason & Chaough. She is now the one dealing with "the kids" and late-night crises, proving she has become the very person Don used to be. The Doorway (1)Mad Men : Season 6 Episode 1
The title refers to more than just physical entrances. It represents: The episode is bookended by the concept of