George Catalin

Your answer clarified the problem. Thank you for your answers @...

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So to clarify this answer:

The rate limit is applied to the account that the app is installed in.

The rate limit will apply to the Zendesk account that granted permission to the third party app and not to the third party app Zendesk account, right? For more clarification, the OAuth token is not needed for Zendesk apps, but for an external service that is processing the data and serving it to a Zendesk support app (browser iframe). The Zendesk support app is not using the OAuth token.

For OAuth authentication a third party app needs a Zendesk account to get a client id.
This third party app needs to manage the data from multiple Zendesk support accounts.
How does rate limiting apply in this scenario? The rate limits are applied to the third party  app Zendesk account or the Zendesk support account?

What authentication type is recommended for managing multiple Zendesk support accounts?
If global OAuth is recommended, the process of obtaining it is slow?