Modifiers: usage of " which " vs " that "

[IndradeepB590]

The Manhattan Sentence Correction book instructs the usage of "which" in case of non essential noun modifiers and " that " in case of essential noun modifiers. However, in an exercise question it states " Regina returned the dress, which was torn at one of the seams, to the store" as correct. The clause " torn at one of the seams " is an essential noun modifier as it identifies the particular torn dress and therefore shouldn't we use " that " in place of " which " ??

Thanks in advance

[RonPurewal]

questions about our materials belong in the "Manhattan GMAT non-CAT Verbal" folder, not here. thanks.