So let's call my PNG image layer Z. I have group A of layers that is behind my layer Z and group B of layers that in front of my layer Z. Now, I want to create a clipping mask to apply it to group A of layers using layer Z silhoutte, how can I do that? In Photoshop it's very easy, you simply click with left button of mouse while holding CTRL on a PNG file with alpha channel and it automatically selects the silhoutte of the image, and then you can use that selection on another layer. But how can you do the same thing in Illustrator?
In Illustrator, you can use the opacity of a raster image as a mask. In my example, ill use stars as the group of vector layers, and this transparent image of a cat as the mask/silhouette.
Now we can use this cat as-is for the mask, but it won't look great. I would recommend using photoshop to turn it into a black and white image, or if you just want the silohuette, play with the levels.
In the Opacity menu of the Control or Appearance tabs, click the button that says "Make Mask" Your menu should now look like the right-hand side.
In the opacity menu, there are now two boxes. One shows the image, the other shows the mask. If you've used masks in photoshop this should be very familiar. Click on the mask square, and paste in your mask image. You may need to move the mask image around, but eventually you should get a result somewhat like the product below.
As you can see, the b&w mask has more detail, but gets lost in a busy background like the stars. The mask with increased contrast paints a more defined edge, but loses all detail.