Account key is created and provided by FI. Account key determines the G/L account to which the amount is to be posted. SD defines account key in pricing and FI uses it for posting taxes. To the maximum both are one and same to find the right G/L account through condition technique .SD Account key will be from Pricing procedure and the FI Account key is from the tax procedure. as far as my knowledge is concerned both the Key purpose is same. Acct keys are defined in FI only, SD people only use them, say in pricing. But ultimately they r used for posting the amounts to relevant GL accounts only