Supreme Commander is a rank given to the leader of the Visitor's Earth and possibly more forces in the original series. The title was originally held by John, but has been held by Charles, Diana, Lydia,Pamela, and Philip in the Second Invasion.
The Supreme Commander's outfit is designated with five stripes over the left breast on the uniform, but he or she, like other ranking officers, are allowed to express their own personal styles. Diana once wore a all white uniform with a glossy black neckline vest during her stint as Supreme Commander. Since Pamela was refered to as a Supreme Commander and she didn't have five stripes it can be assumed, that Supreme Commander may have many levels similar to a General which have stars to determine superiority.