Do you want to merge changes in [doc1] back into [doc2]

[netacct1001]

Sometimes when I open a document that was based on another document, I
get a message asking "Do you want to merge changes in [name of current
doc] back into [name of original doc]?" with 3 buttons: Yes, No, No,
and don't ask again"

My questions are:
1. how do I stop this message? - I never use this feature
2. how does Word know when to ask this question or what the original
doc name is?

Thanks.