> For the complete documentation index, see [llms.txt](https://developer.esw.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.esw.com/inventory/inventory-api/concepts.md).

# Concepts

Core concepts for the Inventory API: Available-to-Sell quantities, delivery-country availability, and choosing between GET and POST.

These pages explain the ideas behind the Inventory API.

| Concept                                                                                             | What it covers                                                                   |
| --------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------- |
| [Available-to-Sell (ATS)](/inventory/inventory-api/concepts/available-to-sell-ats.md)               | What ATS is, what affects it, and how it differs from On Hand stock              |
| [Delivery-Country Availability](/inventory/inventory-api/concepts/delivery-country-availability.md) | How a delivery country determines the inventory applicable to a shopper's market |
| [Decision Tables](/inventory/inventory-api/concepts/decision-tables.md)                             | How to choose between GET and POST and how to treat each response                |


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