One of the most critical parts of the Subsocial stack is the application layer. Without applications, users would have no interface with which to use Subsocial.
Applications connect to Subsocial using the Subsocial SDK, allowing them to read and write to and from the blockchain. These apps can then, if desired, filter the content shown to their users to create a curated experience. For example, an application could be launched to just focus on dogs, and the operators of the app would create a filter so that cats are not shown.
Every app on the Subsocial network has access to the full content and userbase present on the Subsocial network. This means that new applications can be automatically populated with existing content, easing the launch process of a new app. With this in mind, apps will be competing in different market segments to provide users the best interface, experience, and cost.
It is likely that most applications will pay the usage and storage fees (with tokens) for their users. This creates a much smoother user experience as the users do not need to acquire tokens. Applications can still pass on the cost to the users through subscriptions, ads, or just a small fee for every action, but these could be paid with a credit card, another token entirely, or some other method.
Applications could also exist for other uses besides normal social networking. As previously mentioned, an app like AirBnb could be created on Subsocial, which would only show rental listings that were created from that app, cutting out all of the other social noise on the network.
Another aspect that may vary between apps is the level of decentralization. One app may require a crypto wallet and not pay fees, meaning their users have to acquire tokens on their own. A second app might allow you to login with an email and password, and store the private keys for their users. This would be quite centralized, but probably appeal to grandma much more than the other app. They could then offer the future option to transfer all of a user's content and NFTs to a new wallet that a user created, allowing the user to transition to a decentralized style.