It’s a concentrated group of people where people know each other and their tastes. Such group could be a group of housewives doing kitty parties on regular basis. Association of some kind, eg. Traders, elders & senior citizens, shopkeepers, certain category of employed people, Society, NGO or any such organization, Mohalla group, caste / religious groups and all such arrangements. Here a small group of people represent a large section as a subset. Spreading message and expecting good response from this group is convenient and easy for marketing purpose. Ad agency or an Ad network with this kind of group can demand high CPA cost.