> ## Documentation Index
> Fetch the complete documentation index at: https://docs.voucherify.io/llms.txt
> Use this file to discover all available pages before exploring further.
# Stacking rules and categories overview
> Control how multiple incentives interact in a single redemption with stacking rules and categories
When customers use more than one incentive in a single order, stacking rules and categories control how they interact. They define application order, compatibility between campaigns, discount calculation, and limits on how many incentives can apply.
## What you can configure
Use stacking rules and categories to define how campaigns behave when combined.
Create categories, set hierarchy, and define how campaigns behave when combined. Assign campaigns to categories to control stacking.
Configure global settings such as application order, validation policy, discount calculation, and limits.
See practical scenarios that show how stacking rules and categories work together.
## Key concepts
These concepts help you understand how stacking rules and categories work together.
* **Categories**: group campaigns and define how they interact when used together.
* **Stacking rules**: define default behavior for validation and redemption requests.
* **Hierarchy**: defines priority between categories when **Hierarchy priority** is used. A lower number means higher priority.
## Category stacking eligibility
Each category has a stacking eligibility that defines how it interacts with other categories.
* **Common**: follows standard rules. Stacks with **Common** and **Always stackable**, but not with **Never stackable**.
* **Never stackable**: blocks **Common** categories in the same redemption. Still stacks with **Always stackable** and other **Never stackable** categories.
* **Always stackable**: always joins the redemption stack if valid. Stacks with all categories, including **Never stackable**.
## How categories interact
The table below shows how different category types behave when combined.
| Category A | Category B | Result |
| ---------------- | ---------------- | -------------- |
| Common | Common | Stack together |
| Common | Never stackable | Do not stack |
| Common | Always stackable | Stack together |
| Never stackable | Never stackable | Stack together |
| Never stackable | Always stackable | Stack together |
| Always stackable | Always stackable | Stack together |
Use the **Compatibility guide** in the **Stacking rules** view to preview how categories interact before assigning campaigns.
## Why stacking rules matter
Stacking rules help you control how incentives combine and prevent unexpected results.
You can use stacking rules to:
* **Prevent over-discounting**: limit how many incentives apply in one request.
* **Protect exclusive offers**: use **Never stackable** categories to block **Common** campaigns.
* **Ensure required incentives apply**: use **Always stackable** for campaigns that should always be included.
* **Control discount order**: choose between request order and hierarchy priority.
* **Handle edge cases**: define behavior for failed validation, rollback, or no-effect incentives.
## What's next?
To start using stacking rules, first create categories and then configure global settings.
Start with [Manage categories](/orchestrate/manage-categories) to create your category structure. Then configure [Stacking rules](/orchestrate/stacking-rules) to define global behavior. See [Using stacking rules and categories](/orchestrate/using-stacking-rules-and-categories) for real-world examples.