The transparency in images is achieved most commonly by the lack of a pixel or its opacity. In printing, the ink dots represent the pixels and these ink dots have an actual physical form and occupy space, so the transparency on the transfer paper cannot be as detailed as on the screen.

Also when the transparency is applied to a PNG file on an editing software, there is no background but when the sublimation transfer is applied on mostly a white material, like any semi transparent object, it takes the shade of the background that you see through the transparency. you can check the effect by placing the material as a background colour behind the transparent area to see the approx result. 

Please check the image below to see how pixels with 50% opacity looks with and without background. So we can kind of say depending on the transparency level, it is generally the lighter version of the solid colour.

Please keep in mind that this is for sublimation and unlike DTG process it doesn't have a white under base.