Pick-up bars allow you to include music before the first full bar. They are also known as an “upbeat” or “anacrusis”. Often, pick-up bars only comprise a few beats whose main purpose is to lead in to the start of the piece.
Pieces that start with a pick-up bar have time signatures that are positioned at the start of the system as normal. However, the first full bar of the time signature occurs after the first barline and not before. Therefore, pick-up bars do not contribute to the bar number count. Bar numbers are counted from the first full bar in the flow.
Because pick-up bars are linked to the number of notes/rests in the music, in Dorico Pro they are linked to time signatures. However, you can hide time signatures you do not want to show in the music.
You can use the Global Adjustment of Current Bar Insert mode scope to create and delete pick-up bars by deleting notes/rests in the first bar in the flow. You can also use it to shorten the last bar in flows that start with a pick-up bar.