The following parameters may help more precisely locate your research results:

Conjunctions that could be used between keywords for search in the English database
1. “&” means “and”. If you put this mark between two words, it means that these two keywords must occur concurrently in your search results. E.g., “legal & system” means that these two words must exist concurrently in your search result, but it does not necessarily, though could, mean that the result will be “legal system”.
2. “+”: means “or”. If you put this mark between two words, it means that either of the two keywords must occur in your search results. E.g., “legal + system” means that at least one of these two words must exist in your search result, but it does not necessarily, though could, mean that the document can include only one keyword. They may occur at the same time.
3. “ or ”: same as above.
4. “|”: same as above.
5. “ ”: If you put your keywords within quotation marks, it means that you wish to search the database with the keywords arranged exactly in the right sequential order. E.g. “commissioned customs declarations” means you wish so search the database with all these keywords and appear in the word order as shown in the quotation marks.
6. “!” means “not”: If you put this mark between two words, it means that the first keyword must occur in your search results, but the second keyword must not occur. E.g., “legal ! system” means that the keyword “legal” must exist in your search result, but the document of your search result can not include the keyword “system”. They can not occur at the same time.

中文关键词中可使用的连接词：
1. “&”：意为逻辑“与”，指检索结果中必须全部包含其所连结的多个关键词。
2. “ and ”：同“&”。
3. “ ”（空格）：同“&”。
4. “*”：同“&”。
5. “+”： 意为逻辑“或”，指检索结果中将包含其所连结的某一个或多个关键词。
6. “ or ”：同“+”。
7. “|”：同“+”。
8. “!”：意为逻辑“非”，指检索结果中包含其连结的前一个关键词，但不能包含其连结的后一个关键词。