Originally Posted by Rick

it reads both because some people prefer to work in lambda, others in AFR. It' just like a boost gauge can be in PSI or bar for example. Lambda 1 is a stoichiometric ratio. It means that all petrol and all air is consumed in the combustion process with no excess of either.

Rich mixture means excess fuel, and lean excess air. Rich is anything less than lambda 1, lean anything over.

Rick