P: Exposure Selection Tool

Andrew is correct. The real exposure is the amount of light that falls on the sensor. You can't change that in post production. The name 'Exposure' for this slider is a relic from the past. In Lightroom 1-3 this slider did mimic the effect of a change in exposure. If you moved it up, every pixel in the image would become brighter to the same extent, just like a change in exposure does. In Lightroom 4 this changed. The slider now mainly acts on the midtones in the image, as you can clearly see when you look at the histogram while you move it. So a better name would now have been 'Midtones', which would also be more inline with 'highlights' and 'shadows'. It should then also be placed among those sliders, not by its own (with Contrast) at the top.

As I'm imagining it, you could hover over any part of an image whether it's black, white, or any color, and it would show you the (18% grey) exposure of the whole image based on that subject that you're hovering over.

By @tonyc0101

Do you really understand what that means? It means that when you hover over a highlight, Lightroom will show you a grossly underexposed photo, representing what would have happened if you had exposed this image based on this highlight being 18% grey. If you hover over a deep shadow, the opposite happens. Lightroom will show you a completely overexposed photo, representing what would have happened if you had spot metered the shadow and exposed the image as if the shadow was 18% grey. You will only get a useful effect if you hover over an area that ought to be 18% grey, for the reason I already explained: Lightroom doesn't know what the brightness of other areas is supposed to be, so it can't compensate for that.

To use the analogy with the white balance picker: if you use that picker, you must click an area that ought to be neutral grey. You can't click the blue sky and then expect Lightroom to understand that this sky should not become grey as a result of that click, but some other shade of blue.

@tonyc0101 

Why would you do that?!

Let's say I have perfectly exposed my photo: nothing is clipped/lost the entire range of the scene is contained withing the range of the sensor

I can do whatever I want with that info.

If in my photo the shadows are too dark to my taste then I would just them and lift them and leave the rest as it is.

I really don't see the utility of what you suggest.

Changing the entire image with a single setting is the least desirable thing to do.

.

The slider called "exposure" has absolutely nothing to do with exposure. It has everything to do with brightness. Again, despite the name, exposure only takes place at capture and is the sole attribute of how much light (photons) strikes the sensor. 

And again, there are many tools to globally or selectively alter the brightness of the captured image, and again, none that affect exposure. 

@C.Cella As I'm imagining it, you could hover over any part of an image whether it's black, white, or any color, and it would show you the (18% grey) exposure of the whole image based on that subject that you're hovering over.

@TheDigitalDog Wouldn't that be untrue as there actually is an Exposure slider in Lightroom that does exactly that? lol

@tonyc0101  so, just to be clear, with this tool if one clicks on a black or white area on the photo then Lr automatically adjusts/changes exposure so the entire spectrum of pixels or just the blacks/white pixles?

.

Maybe I need to clarify. An exposure selection tool could be used to set (or prioritze) the exposure based on a subject in the image as opposed to adjusting the Exposure slider back and forth until it "looks good" (same as when you use the White Balance sliders). 

By @tonyc0101

It is important to recognize that nothing in the product can affect exposure, that only takes place at capture. You can alter the brightness of some or all of the image and many tools exist to do so. 

You want some area or all the image brightness to look good, there are tools (even one incorrectly called "exposure') to do so until it 'looks good'. There are selective controls too. 

You can only actually prioritize exposure when you set aperture and shutter when you make an image capture, nowhere else. 

@JohanElzenga Correct, yes, an "18% grey picker."

What do you mean with 'as well'? I can only see it work this one way, not any other way because Lightroom doesn't know how bright a pixel is supposed to be. You haven't explained how it would work any other way.