To use any of the Buffer triggers and actions, you need to have a Buffer account. If you do not have a Buffer account yet, you can create one at Bufferapp.com.
To connect your Buffer account to Integromat, follow the general instructions for Connecting to services.
After you click the Continue button, Integromat will redirect you to the Bufferapp.com website and you will be prompted to grant Integromat access to your account:
Here, allow access by clicking the Allow access button. Afterwards, you will be redirected back to the Integromat administration page. Now, you can continue creating your scenario.
If you use the Create a post module to add a new post to your Buffer queue, Buffer will return a unique identifier (ID) for each service. That means that if you publish a post, for example, to three different services (Facebook, Twitter, LinkedIn), then three IDs (three bundles) will be returned. You can then let other modules in your scenario process these bundles.