In the rules for a treaty between major power (MP3), it says that the trade agreement put into place is limited by the "may trade row of the treaty chart". (page 28)
However, I have not been able to find any such information. My treaty chart only shows the effect upon minor countries.
Where can I find this information?
Good spot. I had never noticed this requirement in the rules - probably because, as you correctly point out, there is no such row on the treaty chart. The game seems to work fine without any restriction anyway so my suggestion would be to ignore it.
One thing to note is the difference between trade agreements which can be between any two Major Powers and lending which can only be between allied MPs. Lending can be changed every turn as you want but trade agreements can only be changed by play of MP3 or IPO11 options. For some reason this confused me early on.