OT: Lead or lead

Sorry for raising this topic here, but I need some help from a native english speaking engraving specialist:
In the publication I am preparing at the moment, two different composers use the term “lead” to enlighten the musicians, where the main voice is. One writes “Lead” with a capital “L” and the other one “lead”. What would be the correct way of writing this?

“Lead” is a descriptive term rather than a proper noun, I’d say. So capitalize the staff label and layout name, of course, but lowercase when used in a sentence.