Some third-party vendors support their own version of ST_xxx types on Oracle. For example, a vendor might create its own version of the ST_GEOMETRY type.
To avoid possible conflicts between third-party names and Oracle-supplied names, any third-party implementation of ST_xxx types on Oracle should use a schema prefix. For example, this will ensure that if someone specifies a column type as just ST_GEOMETRY, the column will be created with the Oracle implementation of the ST_GEOMETRY type.