The rules that a gatekeeper sets for the distribution of software applications can, in certain circumstances, restrict the ability of end users to install and effectively use third-party software applications or software application stores on hardware or operating systems of that gatekeeper and restrict the ability of end users to access such software applications or software application stores outside the core platform services of that gatekeeper. Such restrictions can limit the ability of developers of software applications to use alternative distribution channels and the ability of end users to choose between different software applications from different distribution channels and should be prohibited as unfair and liable to weaken the contestability of core platform services. To ensure contestability, the gatekeeper should furthermore allow the third-party software applications or software application stores to prompt the end user to decide whether that service should become the default and enable that change to be carried out easily.