Thank's for all the input, Im'e sure the trick will uncover. But to make clear what I ask, it is using transparant as a color. Like when you use Fill tool on a background and only the background get the fill color, the rest in other color than the background will stand. By the advises untill now I came a few steps further ; Fill do fill transparant, but that cover the intire drawing everything are replaced with a compleatly emty drawing and that was not the idea.
See, I been spending a lot of time using the eraser tool to erase just to the edges of what I want to paste into a new layer on another drawing. That work, but take a lot of time. --- Ithen I wondered if I could fill a new color onto the background, why not do the same with transparant, as pointed out, transparity can be set to zero. But that don't work as expected. At best it fill the intire drrawing with transparant.
From my point of view it would be the perfect tool, becaurse very often I need to paste a figure prepared for this, into another drawing.