If a pixel is transparent, it means that the background takes the place of the pixel. Of course, this can not only be isolated pixels but also entire areas of an image.
You can choose in the Stock List, whether the pixels with the RGB value (0,0,0), (243, 0, 243) or, for example, all pixels with the color that has the leftmost pixel in the top row should be transparent.
In addition, Traffic GIF allows images with transparency. These are images in which the transparent surfaces were already selected during the image processing. This is usually done by specifying in the options when saving, which color should be transparent. But that changes from program to program. For example, in Gimp this is not possible; you have to set an alpha channel, which will then be chosen as transparency.
If you use such modifiers in traffic so that you can place images in front of other images (for example load a car - put the image of the cargo over the image of the car - or make variants of a drawing - only change parts of the original drawing with submitted images) then there are two different types of transparency. One means: the result becomes transparent, the other is: no change in the position. This can only be achieved with such overlay images, which are stored in TVL files, and which have the special “Difference-Mode” community. To put transparent pixels in place of old pixels is done with the methods defined above. No change uses pixels with the RGB value (123, 0, 123).