With sublimation, it is absolutely necessary for the substrate you are printing on to be white or light colored in order to image the substrate.  Sometimes substrates come with just a white patch, which in many cases helps to mask that it is a sublimated item once sublimated.  In some cases, it may be like an iPhone case where the sublimatable piece is an insert and imaging right to the edge of the insert is better.

Most sublimation users will print an image onto the entire white patch to mask the patch after it has been imaged.  There are two reasons why lining up your image with the white patch would be challenging

1.)    It is difficult to measure the exact area of the white patch.  If your measurements are short a millimeter, it will show a thin white  line after sublimating.

2.)    Even if you get the exact measurements, it is difficult to align your image exactly to the white patch

The solution is to bleed your image!

Bleeding also allows you to print all the way to the edge of a sublimatable item!

What is Bleeding?
It is the process of printing your image a little bit larger than the area you plan to print.  This is often used in commercial printing on paper.

How Much Should I Bleed the Image?
Normally a bleed of 0.125 or 0.25 inches for all 4 sides would be enough.  When resizing, be sure to remember to proportionally resize (make sure you increase the size the same amount vertically and horizontally).   Using CorelDraw you can simply add an extra 0.25-0.50 to your vertical or horizontal size.  The reason you have to double the value is because the x or y axis has two sides each.

Safe Printing area
When you are bleeding your image to the edge, it is recommended that you print in the safe area to avoid important elements of the image (such as text) being cut off.  The safe area should be about 0.125-.25 inches within the substrate size.  This is to account for tiny differences in size of your printing area and also to account for human error (in case you printed your image more to the right, left, top, or bottom).   Here’s a diagram below: