A closure is not a wig but an attachment used to close a weave without the use of the natural hair. This is usually sewn in along with weaving or making a custom wig by hand. lace closures and lace frontal closures are the main styles which belong to closures.
Custom units are the other hand are unique. They are specifically made to fit your head. You select the hair, the style and colors you want. Then based on your specific measurements, your custom unit is made for you. If you suffer from hair loss due to cancer or alopecia, this is the best option for you.