1.
Something which is ~, or ~ of, a subject or event, is connected with it or relevant to it. (FORMAL)
All my suggestions ~ the script were accepted...
= with reference to
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2.
Apropos or ~ of is used to introduce something that you are going to say which is related to the subject you have just been talking about. (FORMAL)
Apropos Dudley Moore living in California he said, ‘He loves the space, Californians have a lot of space.’
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