Arrangement is the work of deciding, from a melody and chord progression, which instruments play when and how the song develops, then shaping it into a finished piece. In DTM it covers not only adding parts to tracks but also resting them, separating registers, and varying repeats.
Difference from composing and mixing
Composing creates the core of the song, such as melody and chords; arranging builds how that core is presented; mixing centers on balancing the level and tone of the finished material. They overlap in practice, but separating the roles makes it easier to see which problem you should be solving right now.
It is not just adding instruments
You can create development by adding instruments in the chorus, thinning out the verse, or playing the same chords in a different register. More sounds at once does not always mean more impact. Decide who the lead is and place the other parts so that they support that role.
Building structure along the timeline
In a DAW you lay clips or regions out along the timeline and mark off sections such as intro, verse, pre-chorus, chorus, and interlude. Even when a pattern repeats, changing the fill, the tone, the octave, or the rhythm slightly makes the flow feel natural.
Sorting out register and rhythm
When sounds compete in the same band or rhythm, such as bass and kick or vocal and lead instrument, things feel cramped. Sorting out register, rhythm, and rests at the arrangement stage reduces the need to force parts apart with EQ later. It is also preparation that makes mixing easier.
The order to check things
First listen through the whole song and confirm that each section has a lead and some change. Then try removing tracks and see whether it still works. The basics of the production environment are covered in the DAW article.
