# Concepts (/en/docs/developers/model/concepts)



A model ID selects the model; a protocol defines the request and response format. The platform completes the call through an eligible route.

* [Models and the catalog](/docs/developers/model/concepts/models-and-catalog): model types, protocol support, and billing units.
* [Protocol compatibility](/docs/developers/model/concepts/protocols): different protocols under one API origin.
* [Routing](/docs/developers/model/concepts/routing): route selection, failover, and parameter requirements.
* [Session affinity](/docs/developers/model/concepts/session-affinity): how reusing a route relates to caching.
* [BYOK](/docs/developers/model/concepts/byok): availability of your own provider credentials.

Start with the [quickstart](/docs/developers/model/quickstart) for your first call, or the [catalog](/docs/developers/model/catalog) to select a model.

[API reference →](/docs/api/native/model) · [Cookbook →](/docs/cookbook)