Complex sentence: It can be used for objects that contain flat data (HTML code, SQL query, X.509 certificate...), that is to say, data that can't be easily edited step by step and hence must be edited at once.
Split into simpler sentences: It can be used for objects that contain flat data (html code, SQL query, X.509 certificate...), that is to say, data that can't be easily edited. This type of data can't be edited step by step and must be edited at once.