An application unit for applying a liquid or pasty medium onto a surface moving past the application unit. This has particular application to direct or indirect application of such a medium to a paper or board web. A support beam and a front wall supported generally further from and a rear wall supported generally closer to the moving surface. The walls are spaced apart for defining a pasty medium supply gap between them and are oriented so that the supply gap generally directs the medium toward the surface. The front and rear walls can selectively receive releasable attachment of members for forming a liquid medium an application chamber, the chamber having a downstream end with a blade, an upstream end with a damming strip over which the excessive medium in the application chamber can overflow. Alternatively, a second set of attachable members may be attached to the walls for forming a free jet nozzle dosing gap that communicates with the supply gap for directing the medium to the surface. Therefore, the basic structural group remains unchanged and selectively it may be modified to either an application chamber or a free jet nozzle dosing gap. In an alternate embodiment, the elements defining the dosing gap themselves can receive the attachable elements for defining the application chamber. This enables rearranging the application unit either to define an application chamber or a free jet nozzle with replacement of fewer parts, and generally the parts that are toward the surface.