I see, thanks for the explanation!
With a transform constraint you can either set the mix or edit the mapping numbers to control how much affect the transform constraint has. Setting the mix is easier, but it can be convenient to use the mappings because then you can animation 0 to 100% mix to get the effect you want. Otherwise you have to remember to key the mix eg 0 to 53.76%.
A slider's mix works exactly the same way as the transform constraint, so you can use it to control how much effect you get from the slider. Other ways to change the magnitude of the slider effect is to change the mapping numbers or edit the animation keys or animation duration.
If we had another setting on the slider for controlling how much of an effect you get, what would it do? That sounds like the same thing mix does. If you have an animation that moves a bone, you can set a mix < 100% to move it less or > 100% to move it more.
Here's an interesting setup to think about, magnetic IK!