What is the Data Tier?
The data tier persists dynamic data and provides the primary means of storage for such data as catalogs, campaigns, user profiles, targeting, navigation, business management permissions, state (or session) management, and other site content.
Although the servers in this tier do not communicate directly with clients, you can consider them to be in the run-time environment because they are conceptually an extension of the Web servers. Clients may experience disruptions in service if you do not maintain availability of these servers.
The data tier also provides services for processing transactions, and the aggregation and analysis of business-related data, such as transactions, campaigns, and user profile analysis.