Travel Adjust is the part of the Tx that will allow you to maximize or minimize the amount of servo "travel"...."travel" meaning just how far a servo will rotate from one end to the other.
Dual Rates effect the overall travel which in turn effects control response sensitivity equally throughout the range of that channel. Reducing the Dual Rates reduces the maximum control rate as well as overall sensitivity.
For example, on my planes...I'll go to the Travel Adjustments, first, and set the maximum amount of travel that I can get from one end to the other. Once that is established, that gives me "100% Travel" that I base my dual rates on. Then I'll go to my Dual Rates and dial them down to whatever number I want. On my 48" flying wing, my Travel Adjustment...on both servo's, is 106 for down elevator and 146 for up elevator. Yes, those are weird numbers but that gives me the "100% Travel" that I'll dial down with my Dual Rates.
There is probably a better explanation for what you are asking, and I apologize for using a plane reference to your heli question, but this is how I understand Travel Adjust and Dual Rates.