Most frequently used to visualise an instrument part.
Igor automatically applies all necessary formatting according to the current preferences, inserting headers, instrument names and so forth.
Part Layouts are transposing by default (even if the score is in C), do not display a left barline at the beginning of staves unless there is more than one staff, display cue passages correctly, and consolidate consecutive measures of rests into multi-measure rests. You can override all of these default settings, of course.
