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If the Gap Size is greater than 0, the UV points at the edge of the separated area are additionally shifted so that a gap is created between the new UV island and the surrounding UV areas.
The two images on the left show the original edge or polygon selection. To the right of them are the results after running Disconnect UVs.
Please note that using the Gap Size also shifts the surrounding UV elements. If you want to simply separate the selected elements without changing their point positions, you must use a Gap Size of 0.