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I was corrected in the following sentence:
This message also directs you to reminders about upcoming online events which are either free to subscribers or substantially discounted.

The correction was to change "which" to "that." I'm happy to oblige, but I need a reason. Can you help?
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Hi, maybe Reminders that are Either/or… defines or gives essential (rather than additional) information

Pocket Fowler's Modern English Usage

That may be an alternative to which (and occasionally who). Although they are often interchangeable, there are some uses that are peculiar to each:

(a) When that is used it normally introduces a so-called ‘restrictive’ clause, which defines or gives essential (rather than additional) information about the noun or noun phrase that comes before

(c) Which, not that, has to be used in so-called non-restrictive clauses which give additional rather than essential information: A new edition of the book, which has taken ten years to write, will be published this week.
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It seems that the ' correction ' is not necessary.
Hi, many thanks to whl626 who said, “It seems that the ' correction ' is not necessary”.

This message also directs you to reminders about upcoming online events which are either free to subscribers or substantially discounted.

I agree. Sorry for being careless. The ‘correction’ isn’t needed.

For my own sake, I’ll work through the sentence again.

The noun that comes before ‘which’ is ‘events’. ‘Events’ introduces a non-restrictive clause. The words “either free to subscribers or substantially discounted” give additional rather than essential information.

If the sentence read like this
This message also directs you to reminders about upcoming online events that are scheduled during November…

Then maybe the ‘correction’ would be warranted.