There are various types of add-ins.
A tool add-in makes a visible change to the Survey123 field app. It can automatically open on start up, changing the default behavior of Survey123. In this way, you can control the experience for your users, showing them only the surveys or activities you want them to complete.
Alternatively, you can add tool add-ins to Survey123 as gallery items. These add-ins appear along with surveys in the field app and can be opened in the same way as a survey. This is one way to customize a single survey while keeping the regular Survey123 user experience for other surveys.
Tool add-ins can also be opened by other add-ins (either tool or service add-ins). For example, you can use a map add-in by default when Survey123 opens, and once a user selects a survey from the map to complete, a custom survey tool opens that includes a safety checklist that must be completed before the survey is started.
Service add-ins, such as location tracking and updating data, are not visible in Survey123; they run in the background.
Add-ins can be presented as a tab, tile, or service.
You can customize how question types appear on a form with a custom control.
Custom application input can be used to replace a default question control or to allow input in other parts of the field app.
A scanner add-in uses the default camera component to activate the camera, but instead of returning a picture, it returns text or numeric values derived from the scan.
A custom camera add-in can extend the processing of a picture taken with the camera.
For a full list of components, properties, and methods, see the Field App API Reference.