An archive refers to all the files on a tape. A volume is the files that are written to in one backup session. Thus, if you make one backup to a tape, then it will have one volume. If you then append files to this tape, it will have two volumes. If you append again, it will have three volumes.
For all intents & purposes, volumes are transparent to the user. When selecting files to restore, all the files on all the volumes are presented in alphabetical order (not volume order).