The following schema names are reserved words and must not be used:
In general, you should avoid schema names that begin with SYS to avoid potential migration problems in the future. The database manager will not allow you to create triggers, user-defined types or user-defined functions using a schema name beginning with SYS.
It is also recommended that you not use SESSION as a schema name. Declared temporary tables must be qualified by SESSION. It is therefore possible to have an application declare a temporary table with a name identical to that of a persistent table, in which case the application logic can become overly complicated. Avoid the use of the schema SESSION, except when dealing with declared temporary tables.