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.
Minimum technical standards remain unchanged for all performing courses, so still ABRSM Grade 2 or equivalent for Nat 4, Grade 3 for Nat 5, Grade 4 for Higher and Grade 5 for Advanced Higher, and you’ll still need at least two pieces for each performing study (instrument or voice).
Performance (the skill) and Performing Skills (the unit name) mean what they say… basically learning to play/sing on two instruments (or one instrument and voice) and using the skills acquired to perform (in front of other people, video camera etc.) on them. 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: Performing Skills (National 4) currently still requires you to ‘Perform music in a range of music styles by’:
1.1 Developing musical and technical skills, showing an understanding of the composers’ intentions
1.2 Identifying strengths and areas for improvement in their performing
So the ‘performing’ part’s pretty obvious… you have to perform, and with greater musical accomplishment as well as more technical difficulty as you move up the levels. But separate unit specifications (where applicable) also ask you to evaluate your own performance with similarly increasing awareness. Your performances for these are assessed as pass or fail by the teacher, but your reflections on at least some of them are required for the unit pass. The SQA makes it quite clear that these should relate to technical and musical features and not your experiences, while also stressing that you’re required to reflect once on each performing study (instrument or voice) and suggesting that it’s good practice to do so for all of your performances.