---
title: "Concepts: how the ad server works"
description: "The entry point to the explanation pages — what each concept page answers, in the order a new network manager meets the questions, from what an ad server is through ad selection, serving state, hierarchy and linking, auctions, video, analytics and the permission model."
type: "concept"
audience: [network-manager, publisher, advertiser, admin]
keywords: [concepts, how it works, explanation, ad selection, serving state, hierarchy, linking, auction, analytics pipelines, permission model, where to start]
---

Fetch the complete documentation index at: https://epom.com/ad-server/docs/llms.txt. Use this file to discover all available pages before exploring further. Append .md to any documentation page URL to get its markdown version.

# Concepts: how the ad server works

A guide shows you where to click; a concept page explains why the ad server does what it does,
so that the next problem is one you can reason about instead of one you have to ask about.
Each page below answers one question with worked examples and numbers, then names the
misreadings that cause tickets and links the guides where the behaviour is configured.

**Example.** A campaign is on, its creative is approved and linked, and it still does not
serve. [Serving state](https://epom.com/ad-server/docs/concepts/serving-state.md) lists the eight conditions an impression must
pass, [How ad selection works](https://epom.com/ad-server/docs/concepts/how-ad-selection-works.md) shows where a Non-guaranteed
campaign loses to a Premium one, and [Hierarchy and linking](https://epom.com/ad-server/docs/concepts/hierarchy-and-linking.md)
explains the silent unlink that follows a creative resize. Read in that order, the fix is
usually one setting.

## Start here

| Question | Page |
|---|---|
| What is an ad server, and when do I also need a DSP? | [Ad server vs DSP: what each does and where Epom fits](https://epom.com/ad-server/docs/concepts/ad-server-vs-dsp.md) |
| What happens between an ad request and the ad that wins? | [How ad selection works](https://epom.com/ad-server/docs/concepts/how-ad-selection-works.md) |
| Why does an entity that is switched on not serve? | [Serving state — what makes an entity actually serve](https://epom.com/ad-server/docs/concepts/serving-state.md) |
| How do sites, zones, placements, advertisers, campaigns and creatives fit together? | [Hierarchy and linking](https://epom.com/ad-server/docs/concepts/hierarchy-and-linking.md) |
| Who sees what, and what can they change? | [Sharing and permission model](https://epom.com/ad-server/docs/concepts/sharing-permission-model.md) |

## Money and auctions

| Question | Page |
|---|---|
| What does a winning DSP actually pay? | [First-price vs second-price auctions](https://epom.com/ad-server/docs/concepts/auction-first-vs-second-price.md) |
| What do CPM, eCPM and RPM each measure, and why is the summary row not the average? | [CPM, eCPM and RPM explained](https://epom.com/ad-server/docs/concepts/revenue-metrics-cpm-ecpm-rpm.md) |

## Numbers and traffic quality

| Question | Page |
|---|---|
| Why can two reports over the same dates disagree and both be right? | [How Epom computes your numbers: General, Advanced and raw-log reports](https://epom.com/ad-server/docs/concepts/analytics-pipelines-explained.md) |
| What is invalid traffic, and where does the platform show it? | [Invalid traffic (IVT) and how Epom flags it](https://epom.com/ad-server/docs/concepts/invalid-traffic-and-risk-scoring.md) |

## Video

| Question | Page |
|---|---|
| What are VAST and VPAID, and which one should I traffic? | [VAST vs VPAID: video ad tags explained](https://epom.com/ad-server/docs/concepts/video-vast-vpaid-explained.md) |
| How is video viewability measured on a third-party player? | [Video viewability and Open Measurement (OMID)](https://epom.com/ad-server/docs/concepts/viewability-and-open-measurement.md) |

## Running a network

| Question | Page |
|---|---|
| How does one installation become my branded network, and where is the tenant boundary? | [White-label and multi-tenancy](https://epom.com/ad-server/docs/concepts/white-label-and-multitenancy.md) |
| What does a demo account get, and what does the trial lock block? | [Demo mode — how it works under the hood](https://epom.com/ad-server/docs/concepts/demo-mode.md) |

## How a concept page is organised

Every page opens with what the concept is and why it matters, then **How it works** in order,
**Common situations** worked through with numbers, **Misconceptions and gotchas**, and
**Where to configure it** with links to the guides. Large detail sits on a subpage listed under
the page in the sidebar — for example
[Linking permissions and auto-linking API](https://epom.com/ad-server/docs/concepts/hierarchy-and-linking-reference.md) under
[Hierarchy and linking](https://epom.com/ad-server/docs/concepts/hierarchy-and-linking.md).

## Related

- [Platform reference](https://epom.com/ad-server/docs/reference.md)
- [Epom Ad Server at a glance](https://epom.com/ad-server/docs/start/platform-overview.md)
- [Troubleshooting overview](https://epom.com/ad-server/docs/troubleshooting.md)