Yes, they're 2 different things, but they are intertwined. First you set your control throw maximums int the Servo Setup/Travel menu. These should be the maximum throws you'd ever want from them.
Then your DR controls the percentage of those throws you want depending on the D/R switch settings. Say you had a 3D plane. When flying 3D, of course the control throws are at their maximum. But it would be hard to do some precision flaying, and taking off and landing when the controls are that sensitive. So you set a DR setting with lower max throws for stuff like that. Your control outputs are still linear to the inputs given. So say you want half the ail and elev throws for takeoff and landing. You set the DR to 50% for those with the DR switch in low, or whichever position you choose to be low rates.
With expo, now you can make the output non-linear. So movements of the control stick near the center have less output at the control surface, but as you move the stick farther from center, the surface will move faster and faster until it reaches the max travel you set in DR. Sometimes people use negative expo for quicker output in 3D situations and such.
Both DR and Expo can be, and almost always are, programmed together on the same DR switch, since you usually want them adjusted together anyway. They work together. Like you may use less expo if you DR is down low, but if in 3D mode you may need lots of expo with that max control throw.
I have a Dx8, but your Dx7s should still have a nice screen on your DR/Expo that has a graph, so you can see what your settings do to the output. Move the control you're adjusting and you can even see the stick position input vs servo position output on the graph. Play around with it and it should become clear.
I almost exclusively put all my DR/Expo control on the Ail DR switch. I'm too much of a newb to be having separate DR switches for each channel.