Since National 5, Higher and Advanced Higher courses have all now changed to move completely to external assessment, only National 4 retains the old mandatory unit requirements beyond summer 2019.
Composing (the skill) and Composing Skills (the unit name) mean what they say… basically making up your own music. While the Course Assessment requirements tell you what’s required for an overall course award at all levels and are all you really need to know for Nat 5, Higher and Advanced Higher, Music: Composing Skills (National 4) currently still requires you to ‘Create original music by’:
1.1 Identifying the compositional methods and music concepts used in given examples of music
1.2 Experimenting and using music concepts and compositional methods to develop and create original music
1.3 Developing musical ideas which convey their creative intention
1.4 Reflecting on their music and their creative choices
So the ‘composing’ part’s pretty obvious… you have to compose, and with greater musical accomplishment as well as more sophistication as you move up the levels. But both separate unit specifications and external assignments also require you to demonstrate that you can plan and evaluate your own work with similarly increasing awareness. While unit work is assessed as pass or fail by the teacher where the Nat 5, Higher and Advanced Higher assignments are graded externally, you have some kind of log to complete for either.