System users in AEM 6.5

System User is a JCR (Java Content Repository) user with no password set and minimal privileges required to perform a specific task. System users are typically used by the AEM backend code, which authenticates as a system user when necessary to create, read, update, or delete nodes in the JCR. It is important to remember to log out of a session when the task is complete, as not doing so can cause issues such as flooding the AEM with open sessions, resulting in memory problems and crashes. Take a look at this article for more information on how to programically create a system user in AEM. 

Create System | Service Users in AEM 6.5 with Code Configurations - Sourced Code