Cutis aplasia means ‘missing skin‘ and is a congenital condition where part of the skin (most commonly on the scalp) is missing. This can range from being very small to up to 10cm in diameter. It usually just affects the epidermis, but can affect other skin layers too.
If the missing area of skin is small, it is usually left to heal over some months without any treatment (just gentle washing and keeping it clean). A scab will form and then flake. With larger areas of skin, tissue expansion treatment may be required (a balloon is inserted and used to stretch the healthy skin over the exposed area). Skin grafts are also an option.
References
https://www.gosh.nhs.uk/conditions-and-treatments/conditions-we-treat/cutis-aplasia
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4323704/
https://rarediseases.info.nih.gov/diseases/5835/aplasia-cutis-congenita