There are some decent answers so far, but I think this part deserves a bit more attention. 

 > What would an experience Game Dev understands if I say ´make this Rect a mask´? 


To a game dev, there would be no doubt that if you are using a Rect (short for rectangle in a lot of game engines), that you would clearly be asking about an image mask.  You would generally not be creating a data mask from a poligon.

However, if the developer is unfamiliar with the project, there may be some ambiguity about if you want a vector or raster mask.  2d graphics can generally be saved in 2 ways, a vector image which is a composition defined by lines drawn from points procedurally filled with colors, or a raster image which is an image made from a grid of pixels.  In most cases it does not matter that much which kind of map you use, but it can.  If you are trying to mask an image that has to be highly scalable, you will generally want to use a vector mask to keep from loosing detail to poor resolutions.  If you need to make an especially complex mask, like for doing selective tinting, then a raster mask is often the better choice.