Within the Chord Sheet of Band-in-a-Box®, Part Markers are placed to indicate a new part of the song (they typically occur every 8 bars, but you can place them at the beginning of any bar).

But what else to Part Markers do?

Change the substyle: each original Band-in-a-Box® Style includes 2 substyles: "a" and "b" - there are also MultiStyles available, typically with 4 substyles - but there can be up to 24 ("a" through "x")!
From the Band-in-a-Box® Help file:
Each style has "a" and "b" substyles. Band-in-a-Box® Multistyles also have "c" and "d" substyles, and could have from "a" to "x" for a total of up to twenty-four.

Most of these MultiStyles that we’ve made have 4 substyles, conforming to the following pop song format:
Substyle "a" is usually used for the verse of a song.
Substyle "b" is usually used for the "b-section" or the chorus, and for soloing in the middle choruses.
Substyle "c" is usually used for the intro or for an opening verse or pre-verse.
Substyle "d" is usually used for a break or interlude.

There is always a part marker at bar 1 so that Band-in-a-Box® knows which substyle to begin with. The song continues to play in one substyle until it encounters a new part marker. The substyle will change automatically on second choruses when the "Vary Style in Middle Choruses" song setting is selected.

Drum Fills: a 1-bar drum fill will occur on the bar preceeding the part marker: if you add a part marker to bar 8, the drum fill will occur within bar 7.

Section Paragraphs: adding a part marker to your Band-in-a-Box® song will move that section to a new line within the lead sheet, making it easlier to see - this is optional, and can be adjusted within the Display Options window.

How many part markers do you typically add to your Band-in-a-Box® songs?
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PG Music Inc.