A, B and C1 are the highest three social grades in the most commonly used classification. Media companies frequently use the proportion of their audience in these grades as a measure of the affluence of their audience — and therefore the attractiveness of the audience to advertisers.
This is in many ways a fairly crude measurement. Its main advantages are that it is widely available, well understood, generally accepted and it is easy to gather the data. Because it is commonly available, it is useful for comparing different media businesses.