While Custom Integrations allow Workplace customers to build automation and use APIs for their own community, only Third Party Apps are allowed to act as SaaS integrations that can be installed by multiple Workplace customers.
Third party apps must follow a request and review process before they are made available to customers.
* App Review is: 1) required before your app can be used by other Workplace customers, then 2) repeated annually in the future
† For apps using mid- or high-sensitivity permissions, Security Review is required: 1) before your app can be used by other Workplace customers, then 2) repeated annually in the future
Review documentation for each of these to understand the end-to-end flow for offering a Third Party App to Workplace customers: