How the background is removed
This is a colour-key remover: it compares each pixel with the background colour and clears those within the tolerance. In Outer background mode it starts from the image border and spreads only through connected matching pixels — so a white shirt on a white background survives if it isn’t touching the edge. Colour everywhere removes every matching pixel, which is right for holes and gaps between letters, or a green screen visible through the subject.
Raise tolerance if a fringe of the old background remains; lower it if parts of your subject disappear. Complex photographic backgrounds (rooms, landscapes) need AI segmentation and won’t key cleanly here.
Transparent GIF tips
GIF transparency is on/off — there are no semi-transparent pixels — so anti-aliased edges keep a thin halo of the old colour. Removing a background close to the colour of the page where the GIF will appear hides that halo. For stickers, follow up with Resize GIF to 512 px; to place the cut-out on a new scene, replace the background with a colour here, or combine it with other GIFs.