HONG KONG, Sept 18 (Reuters) - The disappearance of Chinese Defence Minister Li Shangfu highlights the opaque and complex nature of the People's Liberation Army (PLA), diplomats and analysts say. Reuters reported on Friday that Li is under investigation over the corrupt procurement of military equipment during his previous role. Eight other senior officials are also being investigated. His fate has not been officially explained. HOW POWERFUL WAS LI SHANGFU WITHIN THE PLA SYSTEM? In the Chinese system, the Minister of National Defence is seen as significantly less powerful than the U.S. defence secretary and many international equivalents. The position...