Countries often require an agreement—or commitment—in order to agree to reduce trade barriers bilaterally. Also, tariffs are not always welfare reducing. If the country is large, a tariff may even generate increased social welfare.)
Trade creation would include any new trade created (not existing before the FTA) between China, Brunei, Indonesia, Malaysia, the Philippines, Singapore and/or Thailand.
Trade diversion would include any trade diverted because of the FTA. That is, trade that previously existed between one of the FTA member countries (e.g., China) and a non-member country (e.g., India) that was diverted from India to intra-FTA trade between FTA member countries as a result of the agreement.