SAP R/3: Past, Presence and Future , The Integrated R/3 Repository, Enterprise Modeling with R/3, Objects of the R/3 Data Dictionary, Programming with ABAP/4 and the DynPro Concept, Customizing R/3: Concepts and Techniques, Process and System Architecture.

Class A: Application table (master or transaction data). Maintained by the customer using application transactions. Supplied empty. Example: material master.

Class C: Customer table. Maintained only by the customer. Supplied with SAP model entries. Tables of this delivery class are never overwritten when upgrades are performed. Example: company code tables.

Class G: Customer table with protected content. Like class C, but SAP can add new entries. Existing customer entries are not overwritten. Supplied with SAP model entries. Example: number ranges.

Class L: Table for storing temporary data which can be interpreted as intermediate results with limited time validity. Supplied empty. Example: employee groups.

Class E: System table (control table) with separate name ranges for customer entries. The customer entries remain unchanged when upgrades are performed. Example: special stock indicator.

Class S: System table of program nature. Maintained only by SAP. Table is filled with system settings on delivery. Example: codes for SAP transactions.

Class W: System table for system operation and maintenance. Contents are maintained directly by maintenance transactions. Example: function module table.
Source: www.sts.tu-harburg.de