Hi, user-defined signatures and predefined signatures have same priorities. When traffic matches both user-defined and predefined signatures, the more strict signature prevails.
For more details, please refer to the product documentation:
http://support.huawei.com/hedex/hdx.do?docid=EDOC1100068406&id=EN-US_TOPIC_0134868311&text=Limitations%252520and%252520Precautions%252520for%252520Intrusion%252520Prevention&lang=en