Sequential means that the order of products in the line stays the same throughout the whole process. Products stay in sequence and can be accuretely traced. This is very important when handling food (amongst other things). If for example a certain batch has to be recalled due to contamination its necesarry to be able to trace it. Alternatively, non-sequential means that products do not follow a strict production order. Products could switch places and be handled in a different time or order than originally produced (due to offline buffering for example). Examples could be the printing of magazines, or toilet paper etc...