I don't really know what the X mark signifies, but I've found it's not only me that thinks it means the sentence is incorrect. Correctors often just want to provide a better wording and not necessarily the sentence is ungrammatical, or sometimes only want to comment on it.
I don't really know what the X mark signifies, but I've found it's not only me that thinks it means the sentence is incorrect. Correctors often just want to provide a better wording and not necessarily the sentence is ungrammatical, or sometimes only want to comment on it.