Meaning of maximum load function generator can drive
Most of the function generators' output impedance is \$50\Omega\$ and they give rated terminal voltage only if they are terminated with a \$50\Omega\$ load. So any other load connected to function generated gives amplitude different from that of the displayed.

Some function generators support 'High-Z' mode in which displayed voltage is delivered to the load if the load resistance is greater than a minimum rated value. But I guess there is no upper limit for the load resistance in such cases. You will get undistorted output if you connect a \$60M\Omega\$ resistance. Where did you see this?