The objective criteria are important indications about the market condition back in those days. However, they are not essential to close negotiations but they will certainly help you to improve the agreement.
The objective criteria are hidden in the different scenarios of Venice. They are indicated with an exclamation mark (!)
Once you click on the characters or read the information that appears when clicking on posters, books, contracts, etc., those objective criteria will appear in the list within your proposal.
Take in consideration that in this list you will only see those that you have gathered so far. You will have to choose the ones that suit best with the negotiation you are in.
You should only use objective criteria from the third negotiation onwards.