A system for a drug delivery device (1) is provided, comprising: a housing (10, 350), a cartridge unit (300), which is attachable to the housing or releasably attached to the housing, wherein the cartridge unit comprises a cartridge unit guide feature (327), the cartridge unit guide feature being provided to establish a guiding interface (327, 351) with the housing in order to guide relative movement of the cartridge unit and the housing with respect to one another when attaching the cartridge unit to the housing, and wherein the cartridge unit comprises a cartridge unit interface feature (329, 334), the cartridge unit interface feature being provided to form a further interface, in addition to the guiding interface, with the housing the further interface being established when the cartridge unit is attached to the housing, wherein the guiding interface is a bayonet-type interface which defines at least two different stages of movement between the cartridge unit and the housing when the cartridge unit is attached to the housing, a first stage (351a) with at least axial movement by a first stage axial distance in a first stage axial direction and a second stage (351b) with at least rotational movement by a second stage angle in a second stage rotational direction, and wherein the further interface is or comprises a ramp interface.
