A label category is an overarching structure that consists of a group of labels that all have a common element. A label category can for example be “Client”. The labels within that category would all be clients (Client A, client B, client C, …). In the category “Maintainer” you’d put all your maintainers, and in the category "Lessor" operators can add all the lessors they're leasing locomotives from.
As a Railfleet administrator you can create and modify label categories in the administration panel.