TFN is private between you and the ATO and can impact the tax removed from any dividend payments if not given to 'anyone' an SMSF transacts with.
ABN not really the same problem but its all public information (www.abr.business.gov.au or www.asic.gov.au) and you can't hide by not providing it and there is no incentive to not hand it over.. There's no tax implication with ABN's.
