The SAP system categorizes users into several types for different purposes as shown in the table below:
Type Purpose Dialog Individual, interactive system access. System Background processing and communication within a system (such as RFC users for ALE, Workflow, TMS, and CUA). Communication Dialog-free communication for external RFC calls. Service Dialog user available to a larger, anonymous group of users. Reference General, non-person related users that allows the assignment of additional identical authorizations, such as for Internet users created with transaction SU01. No logon is possible.
Source : SAP Offical Documents