As a new SA user, I've struggled with the difference between Alpha, Mask, and Selection in SA. I'm sure these concepts are old-hat to all the seasoned users here, but as implemented by SA, they are new to me. The following is my own interpretation, not officially "Studio Artist" info ...
Mask and Selection are related. You can turn on Mask, and see your Selection (if there is one), in the clear parts of the Mask.
Alpha is something else entirely, but Alpha can be written to the Mask, which creates a Selection.
There are many ways to create an Alpha in SA (soon to be explored by me), but as a noob, I've been creating an Alpha channel in Photoshop and opening a New Source Image and Canvas in SA, then turning Mask view on, and Canvas : Selection : Set To : Source Alpha. This causes the source image's Alpha to become the Mask (with implicit Selection: the clear parts) in SA. If you want to invert what the mask operates on in the canvas, Canvas : Selection : Invert.
This is a simple concept that I didn't grasp right away, but it helps a lot to understand it clearly.
Each Studio Artist layer consists of 4 image channels, so each pixel in a layer is composed of ARGB, where A = alpha, R = red, G = green, and B = blue. The 3 RGB color channels define a color image. When i say channel, think of a black and white image.
The alpha channel can define a mask associated with the layer. Another way to think about an alpha channel is that each alpha pixel define the coverage associated with it's RGB pixel element.
Or, the alpha channel of a layer can just be used as an extra 1 channel image attached to the layer, to be used however you wish to use it, without having it affect the canvas view at all.
Some digital imaging program like Photoshop for example always have alpha active for masking a layer. So the alpha channel associated with a layer is always working as a mask for that layer, end of story. Studio Artist is more flexible. You can use the alpha channel in that way, but you can also use it more flexibly if you wish. You might use it to store a stencil or as an image buffer for constructing some image effect that needs an extra 1 channel image buffer.
You can configure the canvas view to be alpha enabled, so then the alpha channels associated with the layers ask as masks for the layers. Or you can turn off alpha view, so then the contents of the layer's alpha channels do not directly affect the canvas view display.
The same goes for the paint synthesizer painting into the alpha channel. You can enable the paint synthesizer to paint into the alpha layer, or you can turn that off and then painting will only affect the RGB channels of the layer and not affect the contents of the alpha channel. If you were using the alpha channel as a fixed stencil, or as a buffer for some special effect, then having painting not affect the alpha channel could be useful.
The selection buffer is not directly associated with the canvas layers. You an think of it as a separate raster image buffer. Typically, you use it to define region selections associated with specific areas of the canvas. By turning on Masking you can then mask Studio Artist actions based on the contents of the current selection buffer. So you could use the lasso selection tool to draw an selection around some area of the canvas, then turn on masking and run an image processing effect and the image processing effect would only be applied to the selected area of the canvas.
Again, Studio Artist gives you the ability to be quite flexible in terms of how you use the selection buffer. You can use it as a temporary image buffer for some special effect that has nothing to do with selection per say. For example, you can use the contents of the selection buffer to modulate an interactive warp. I use this trick a lot, setting the selection buffer to be the source image, or to be a noise field, and then using the Selection Modulate Rotate or Selection Modulate Translate interactive warp functions to adaptively warp the canvas based on the ceontnets of the selection buffer.