I am putting a formula in a default value of a field in a transaction.

I want the formula to referance different fields depending on if the transaction is an opportunity or a quote.  I guess I can use Case Where for this but what I need to know is if there a variable I can referance for this, like trantype or simular.

I thought I could do it based on if the subtotal field was null as this doesn't exist on the opportunity but if I refereance subtotal I get field not found error on the opportunity.

Thanks in advance