Instrumentation concerns the combinations of musical instruments that are used throughout a musical composition. It can refer to the comprehensive list of instruments used in a work and to how specific instruments are employed in a section of the work.
Orchestration is sometimes used as a synonym for instrumentation. However, it is a somewhat narrower term, because it is frequently used to describe the art of instrumentation as related to the symphony orchestra, whereas instrumentation is the art of combining instruments in any kind of musical composition, including such diverse types as chamber music, marching bands and non-Western music. Orchestration can also mean transcribing for orchestra of a work originally written for a solo instrument or a small group of instruments.
Good instrumentation requires that composers and arrangers have an in-depth understanding of the characteristics of each of the specific types of instruments they employ, including their ranges of pitches, timbres or ranges of timbres, dynamic ranges, difficulties in playing certain types of passages (such as very long, or rapid, or repeated notes, or glissando), how multiple instruments of the same type interact, how multiple instruments of different types interact, how the various sections of the orchestra interact with each other, and specific notational conventions.