What's the difference between 扣好安全带 (kòuhǎo ānquándài) and 系好安全带 (jìhǎo ānquándài), which both seem to mean "fasten seatbelt"?
扣 means to " to hook / to buckle" 
"扣好安全帶" means "buckle up the seatbelt "

系 means "to tie /to fasten" 
"系好安全帶" means to " fasten the seatbelt " 

Although there are many ways to fasten a belt to something, when it comes to seatbelt, the only way to fasten it to the seat, is to buckle it up. Therefore, 系好安全帶 and 扣好安全帶 are essentially the same thing.