The pocket container selection lets you select which custom containers can be opened from a pocket container. If you have 2 custom containers with the names pulverizer and smithing station, it should look like this:

• Clicking Cancel will take you back to the pocket container edit menu without keeping the changes you made in this menu.
• Clicking Apply will take you back to the pocket container edit menu and keep all the changes you made here.

All custom containers that are currently selected are green and all custom containers that are currently not selected are gray. Clicking an unselected container will select it and clicking a selected container will deselect it. When a player right-clicks with this pocket container, he will be able to choose from any of the selected (green) containers. If only 1 container is selected, the player will open that container right away.