> ## Documentation Index > Fetch the complete documentation index at: https://documentation.nozle.dev/llms.txt > Use this file to discover all available pages before exploring further. # Core Concepts > Plans, subscriptions, events, entitlements, and margin The building blocks of Nozle. Each concept maps to a specific part of the platform -- understand these and the rest of the docs will click. ## Plans A plan is a pricing tier -- Starter, Growth, Scale, Enterprise, or whatever you define. Each plan bundles three things: * **Base price** -- a flat recurring charge (monthly or annual) * **Usage-based charges** -- metered fees tied to event codes (e.g., \$0.002 per API call) * **Feature entitlements** -- what the plan unlocks (analytics access, SSO, custom models) Plans are configured in the Nozle dashboard and served to your app through the API. ## Subscriptions A subscription assigns a customer to a plan. When a customer checks out, Nozle creates a subscription that controls what they can access and how they are billed. Subscriptions support: * **Upgrades** -- move to a higher plan mid-cycle with prorated charges * **Downgrades** -- move to a lower plan, effective at the next billing period * **Cancellation** -- end the subscription with prorated credits ## Events Events are usage data points sent from your application via the SDK's `track()` method. Each event contains: | Field | Description | | ------------- | ------------------------------------------------- | | `customer_id` | Who generated the usage | | `event_code` | What was used (maps to a billable metric) | | `properties` | Metadata -- model name, token count, region, etc. | The event pipeline flows: **SDK** → **Nozle API** → **aggregation and billing**. Events drive both usage-based charges and entitlement consumption (counting toward limits). ## Entitlements Entitlements define what a customer can do based on their plan. There are two types: **Feature gates** -- boolean access checks. Either the plan includes a feature or it does not. ``` can("analytics") // → true or false can("custom-models") // → true or false ``` **Usage limits** -- numeric caps that reset each billing period. ``` can("api-calls") // → { allowed: true, remaining: 8420 } can("team-members") // → { allowed: false, remaining: 0 } ``` Entitlements are checked via the [`can()` endpoint](/guides/entitlements/can-endpoint) (REST) or the `useCan()` hook (React SDK). Updates propagate in real-time over WebSocket -- no polling required. ## Margin Margin intelligence is what separates Nozle from a billing system. Nozle tracks the **cost** of each usage unit -- per LLM model, per token type, per provider -- and compares it against what you **charge**. This gives you: * **Per-customer margin** -- who is profitable, who is not * **Per-model cost tracking** -- which models eat into margin * **Alerts** -- get notified when margin drops below a threshold Margin data is computed by the Nozle API and returned alongside billing data. ## API Keys Nozle uses two types of API keys to separate client-safe and server-only operations: | Key prefix | Name | Use case | | ---------- | ----------- | ------------------------------------------------------------------------------------------------------------------------------------------ | | `pk_` | Publishable | Safe for client-side code. Used by the React SDK for entitlement checks, checkout, and plan display. | | `sk_` | Secret | Server-only. Used for sending events, managing subscriptions, configuring cost models, and reading margin data. Never expose in a browser. |