service a

Hi everyone, I have the Mercedes Benz CLK 2.7 CDI 2003 and my board computer just give me the alert that my car is due service A. What does that mean? Is this oil change, filter, etc..? Or something else? Thanks a lot. B

C63 AMG 2009 dash indicator says A5 service due. Mercedes "my service" says B service + brake fluid and plugs as extra when I try to get a quote/book a service. Local Mercedes dealership on-line booking system says A service required but no info on further requirements like plugs or brake...