I gathered that what Tess_Linn was asking was if it was possible to work solely on her selected area without the need for a mask to implement transparency options. Not knowing what the intended use was I simply offered up possible options and methodologies for area selecting which could work just as well and even if you are working on different layers you can always provide an adjustment layer at the top of the stack where all the lower layers can still benefit from the use of curves, levels, hues & saturation, etc. Or you can just isolate one layer. Please note that I was not offering a work flow process and my examples here should not be interpreted as such. As I’m sure has already become evident is the fact that there are many ways to achieve the same results in Photoshop. As you become more experienced and proficient using the software you will no doubt develop your own way of doing things. In that respect the program is just beautifully that flexible. As a side bar Tess_Linn, you can also create a clipping mask by holding down the Alt key while placing the mouse cursor between the layers and clicking when you see the cursor turn into a double bubble.