There are two general kinds of adaptation: transcription, which closely follows the original piece, and arrangement, which tends to change significant aspects of the original piece. In practice, however, the terms transcription and arrangement are often used interchangeably. Orchestration applies, strictly speaking, only to the orchestra, whereas the term instrumentation applies to all instrumental groups. Instrumentation in this sense subsumes orchestration. In the study of orchestration — in contradistinction to the practice — the term instrumentation may also refer to consideration of the defining characteristics of individual instruments rather than to the art of combining instruments.
In commercial music, especially musical theatre and film music, independent orchestrators are often used because it is difficult to meet tight deadlines when the same person is required both to compose and to orchestrate.
Film orchestrators often work from a short score (that is, a score written on several musical staves). Broadway orchestrators are more likely to work from a piano score (as does Jonathan Tunick when he orchestrates Stephen Sondheim's songs, for example) or from a lead sheet. In the latter case, arranging as well as orchestration will be involved.