You can use some pre-defined column values in SELECT statements and as a default value for a column definition. The Rules Engine will replace these values.
Here is a list of pre-defined column values that you can use:
Furthermore, you can use the USER pseudo column (without $$ characters) in SELECT statements.
The USER and $$USER$$ pseudo columns return the active user, and the $$RDBMSUSER$$ pseudo column returns the current RDBMS user. Often the active user and the RDBMS user are the same. However, in web applications for example, you can log on with a specific username, whilst the username which is used to go to the database can be a different one.