Any cropping in ProSelect is not permanent, it just temporarily defines the edges and aspect ratio of the image and is totally independent of the size of the image.
For example, when you crop an image to an 8x10 ratio you are not creating an 8x10 sized image just an image with the aspect ratio of an 8x10 - ie 1:1.25.

When you use ProSelect in Select Size View it will project that image at each one of the setup Print Paper Sizes. By default, these are 5x7, 6x9, 8x10 11x14 etc.

So if you have cropped your image in the 8x10 ratio and display it at 5x7 size, it will actually be shown at 5x6.25 (5 x 1.25 = 6.25), 6x7.5 (9 x 1.25=7.5), 8x10 (8 x 1.25=10) etc.

Working With Layouts

When "working with layouts", the Crop In button allows you to effectively "zoom in" to position a more closely cropped image wthin an opening in the layout.

Once you have cropped in, you can then crop back out but only to the point when either of the edges of the image are aligned with the edges of the opening. (If you where able to crop out further than this, then there would be a gap between the edge of the image and the edge of the opening.)

When you first drop an image into an opening, it is automatically sized and positioned so that either the top or side edges of the image align with the top or side edges of the opening.

For example, if you drop a portrait orientated image into a landscape orientated opening, the sides of the image will match the sides of the opening with the top and bottom of the image extending above and below "behind" the opening. In this case, you can click and drag the image (while in Crop View) to slide it up and down but not sideways because there are no parts of the image extending out beyond the sides of the opening.

Hint:
Hold the Alt/Option key down while using the Crop In and Crop Out buttons to change the amount of change by a smaller amount.