In popular music an arrangement is a setting of a piece of music. An arrangement may add details omitted by the composer, or it may replace those originally given and be merely based on the original work. In classical music an arrangement is a full score, and often a full orchestration.
As with composition, the ready availability of sound recording equipment has changed the understanding of what arrangement means. At one stage, credit for an arrangement would only be given to a person who had produced a written musical score or written chart of some sort. More recently, any original treatment of an existing work that is available for repeated performance by other players may qualify to be loosely called an arrangement. Every time a piece of music is performed it has an arrangement, which may or may not have been done by a professional arranger.
An arrangement may specify or vary some or all of harmonies, including parts, instrumentation, style, dynamics, and other instructions to the players. An arrangement also involves sequence, including the order and number of repeats of sections such as verses and choruses, and provision of sections to be improvised by instrumentalists, as well as introduction, coda, modulations and other variations.
An arrangement is often an adaptation of a given composition into a form other than as originally composed. This includes arrangements for different instruments. For example, an arrangement for piano, or flute, or a duet, may be based on a symphonic piece. It could also be an arrangement of instrumental accompaniment for vocal music. Or, it may be an adaptation for another musical style, for example adaptation of a classical piece for a jazz or rock ensemble, orchestration of a song written by a popular band, or an a capella setting of a song from a stage musical or an opera.
This will explain the name “Arrange Window,” as found in DAWs – the place where the song is composed , instruments are selected, and the order and number of verse, chorus bridge, instrumental solos and etc., is established.