What 9-slice scaling is
A 9-slice (or 9-patch) image is cut into a 3 × 3 grid by four lines. The four corners are drawn at their real size, the top and bottom borders stretch only sideways, the left and right borders only up and down, and the middle fills the rest. One small frame can then be any window, dialogue box or button without blurry corners or fat borders. Every major engine supports it, but each one wants the four numbers in its own place and order.
Finding the slices
The tool looks for the longest run of columns and rows that are identical, which is the part that can stretch without changing the look, and puts the guides at its edges. Ornate frames with no flat part get guides at a quarter of the size. Drag the guides if you want more of the frame kept at its real size. Choose Tile for patterned borders such as stone, rope or wood planks, so they repeat instead of stretching.
Using the values
Godot uses a NinePatchRect with patch margins. Unity stores the border on the sprite in the order left, bottom, right, top, set in the Sprite Editor, and an Image component set to Sliced. Phaser 3.60 and later has a NineSlice game object. On the web, border-image with the same four numbers does the job. Copy the line you need from the boxes under the preview.
Is my image uploaded?
No. Everything happens in your browser.