In the context of marketing, 'on message' means sticking to/staying focussed on and delivering the official, planned or required line of the organisation.
"Why should I buy my computers from you?" might trigger a variety of responses but in order for the sales rep' to stay 'on message' he might be required to say, "Our systems have the best support back-up in the country."
'Spin' would mean to apply a bias, or subjective (often very creative)interpretation or 'twist' to something to show the the organisation/its products in a good light - or, conversely, its competitors in a poor light.
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