A folder in Pine is just a file whose name is the same as the folder you see in Pine. In UNIX Pine, the default place for folders is the "mail/" directory. There is no unique way to store messages inside a folder, although all of the formats agree on leaving the last message in that file at the bottom of the file. This way of sorting a folder is called "Sort by Arrival", because it's the order in which they are saved to the folder.