If I have a pixel alpha value of 1, and on another image the same value of 1, adding them will result in a value of 2 (1 + 1 = 2) this value is not very ideal, then the mode "Plus" is not the most recommended and the "over" operation can also introduce problems using the operation "max", values from the same pixel position on the two images are compared, and it will keep the highest value. 