In Dorico, an instrument is an individual musical instrument, such as a piano, a flute, or a violin. Human voices, such as soprano or tenor, are also considered instruments.
In Dorico, instruments are held by players, just as real instruments are held by human players. Section players can only hold one instrument but single players can hold multiple instruments. This allows you to handle instrument changes easily, such as when an oboist doubling the cor anglais switches from one instrument to the other.
Each instrument automatically gets its own staff, but when instrument changes are allowed, the music for multiple instruments held by the same single player can appear on the same staff as long as no notes overlap.
Dorico has a database of information about the properties of each instrument. These include the playable range, common and uncommon playing techniques, notational conventions, transposition properties, tunings, clef, number of staves, type of staff, and so on. Having these properties predetermined makes it easier and quicker to set up projects correctly. For example, there is a Horn in F instrument that automatically shows treble clefs in transposed pitch layouts. Similarly, there is a timpani instrument that automatically hides all key signatures.