> For the complete documentation index, see [llms.txt](https://bert.gitbook.io/bert-documentation/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://bert.gitbook.io/bert-documentation/getting-started/creating-your-first-system/step-4-define-output-interface.md).

# Step 4: Define Output Interface

## How Does Your System Deliver Its Output?

You've identified what your system produces and where it goes. Now you need to understand HOW it gets there - through what mechanism, process, or channel does your output actually leave your system?

This delivery mechanism is called an **interface** - the specific way your system connects with its environment to transfer outputs.

## Creating the Interface

When you created your output flow in Step 3, you may have noticed it ends with a small rectangle at the system boundary. This rectangle becomes your interface.

**Click on this rectangle to define your output interface:**

<figure><img src="/files/C2yLXZ7qzmqCdTcPi8Tu" alt="" width="244"><figcaption></figcaption></figure>

## Defining Your Interface

Click on the interface element to specify how your system delivers its output:

**Name** - What do you call this delivery mechanism? Examples from our model library:

* **Cell**: "Cell Membrane Transport Proteins"
* **Ecosystem**: "Predation Interface"
* **Solar Panel**: "Electrical Output Terminals"
* **Organization**: "Sales Interface" or "Service Delivery"
* **LLM**: "Response Generation API"

**Description** - How does this interface actually work? What makes it effective at delivering your system's output to its environment?

**Protocol** - What rules or algorithm does this interface use to decide what goes out (and potentially what comes in)? This is the decision-making logic that governs the interface's behavior.

<figure><img src="/files/9xaFnsMYaB2A42bWnMN6" alt="" width="375"><figcaption><p>Interface details panel in v0.2.0</p></figcaption></figure>

## Understanding Interface Protocols

Think of a protocol as the "brain" of your interface - the set of rules that determine:

* When to allow output to flow
* What quality or quantity standards to maintain
* How to handle different types of requests from the environment
* What feedback to send back to the system

**Examples:**

* **Sales Counter**: "Process orders during business hours, verify payment before releasing products"
* **Cell Membrane**: "Allow glucose in when internal levels are low, expel waste when concentration exceeds threshold"
* **Website API**: "Authenticate users, rate-limit requests, return error codes for invalid queries"

Interfaces are where your system meets the world. They're not abstract - they're the actual mechanisms, channels, or processes that make your outputs available to those who need them.