As mentionned above, a mail folder is a collection of mail messages stored in a single file. elm folders are stored within the directory defined by the F)older directory field in the option menu (or the maildir variable in mailrc). The default is $HOME/Mail.
The folder is referred to as
+folder_namewhere the plus (+) sign means that the file is residing (or is to be residing) in the folder directory. If the plus (+) sign is not used, the folder will reside in the directory from which elm was invoked.