They are very similar in appearance, The Replace text comment is a 2-in-1 tool and appears blue by default, and the Striketrhoughis a single function tool and appears red by default. You can change both of these color of course.
While Acrobat shows the Replace Text comment as a single item in the Comments pane, other PDF readers may show it as two separate comment types, as shown below in this screenshot from Bluebeam Revu. This illustrates how Replace Text is a 2-in-1 tool.
It's a good idea to use each tool as it was intended. That makes it more clear what your intentions are when the end-user of your PDF goes to incorporate those comments into the master document. Also, some programs/plugins can utilize the comments from these separate tools to atuomatically apply those changes (into InDesign documents, for example).
Sadly, many users are not very particular about how they use the PDF commenting tools, and use exclusively sticky notes for all types of comments, and then inside each sticky note they describe what they want done. It's much clearer to simply use the proper tool for the job. Your editor will thank you!
So use the Replace Text tool when you want to replace text and use the Strikethrough tool when you want to delete text.
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