This document is a reference for custom activation strategies. If you're looking for a guide on how to use them, see the [_how to use custom strategies_ guide](../how-to/how-to-use-custom-strategies.md).
Custom activation strategies should be considered an advanced feature. In most cases [strategy constraints](https://docs.getunleash.io/reference/strategy-constraints) provides enough control without the additional complexity.
**Custom activation strategies** let you define your own activation strategies to use with Unleash. When the [built-in activation strategies](../reference/activation-strategies.md) aren't enough, custom activation strategies are there to provide you with the flexibility you need.
- A strategy **name**: You'll use this to refer to the strategy in the UI and in code. The strategy name should make it easy to understand what the strategy does. For instance, if a strategy uses contact numbers to determine whether a feature should be enabled, then _ContactNumbers_ would be a good name.
- An optional list of **parameters**: The parameter list lets you pass arguments to your custom activation strategy. These will be made available to your custom strategy implementation. How you interact with them differs between SDKs, but refer to the [Node.js example in the how-to guide](../how-to/how-to-use-custom-strategies.md) for a rough idea.
The strategy **name** is the only required parameter, but adding a good **description** will make it easier to remember what a strategy should do. The list of **parameters** lets you pass data from the Unleash instance to the strategy implementation.
Parameters let you provide arguments to your strategy that it can access for evaluation. When creating a strategy, each parameter can be either required or optional. This marking is to help the user understand what they need to fill out; they can still save the strategy without filling out a required field.
Each parameter has one of five different parameter types. A parameter's type impacts the kind of controls shown in the admin UI and how it's represented in code.
The below table lists the types and how they're represented in the JSON payload returned from the Unleash server. When parsed, the exact types will vary based on your programming language's type system.
All values have an **empty string (`""`) as the default value**. As such, if you don't interact with a control or otherwise set a value, the value will be an empty string.
While custom strategies are _defined_ on the Unleash server, they must be _implemented_ on the client. All official Unleash client SDKs provide a way for you to implement custom strategies. You should refer to the individual SDK's documentation for specifics, but for an example, you can have a look at the [_Node.js client implementation_ section in the _how to use custom strategies_ guide](../how-to/how-to-use-custom-strategies.md#step-3-a).
The exact method for implementing custom strategies will vary between SDKs, but the server SDKs follow the same patterns. For front-end client SDKs ([Android](/docs/generated/sdks/client-side/android-proxy.md), [JavaScript](/docs/generated/sdks/client-side/javascript-browser.md), [React](/docs/generated/sdks/client-side/react.md), [iOS](/docs/generated/sdks/client-side/ios-proxy.md), [Flutter](/docs/generated/sdks/client-side/flutter.md)), the custom activation strategy must be implemented in the [Unleash Proxy](../generated/unleash-proxy.md).
When implementing a strategy in your client, you will get access to the strategy's parameters and the Unleash Context. Again, refer to your specific SDK's documentation for more details.
