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<h1><span class="heading-subtitle">Technical Design Documents</span>Technical Design<br> Documents</h1> 
<p class="lead">The technical and operational specifications of the Once-Only Technical System, laid down in the Commission’s <a class="link external" target="_blank" href="https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=uriserv%3AOJ.L_.2022.231.01.0001.01.ENG&toc=OJ%3AL%3A2022%3A231%3ATOC"><span>Implementing Regulation ((EU) 2022/1463)</span><span class="sr-only">(opens in a new tab)</span><span class="ico-external"></span></a> setting out the system, are complemented by detailed, non-binding technical design documents jointly prepared by the Commission and Member States.</p>

Main chapters

<p>This is the v1.2.1 release of the Technical Design Documents from April 2025.</p>

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<p><small>Chapter 1</small></p>
<h3 class="h5">Introduction - High Level <br>Architecture</h3>
<p>This chapter introduces the high-level architecture of the Once-Only Technical System and the key concepts and themes found in the subsequent chapters.</p>
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<p>Release date: <strong>April 2025</strong></p>
<p><a class="btn primary" target="_blank" onclick="_paq.push(['trackEvent','conversion','oots-tdd','button-view-chapter-1'])" href="https://ec.europa.eu/digital-building-blocks/sites/x/oRqlNQ"><span>View chapter</span><span class="sr-only">(opens in a new tab)</span><span class="ico-external"></span></a></p>
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<p><small>Chapter 2</small></p>
<h3 class="h5">User Identification, Authentication and Record Matching</h3>
<p>This chapter explains how those submitting or receiving evidences through the Once-Only Technical System are identified and authenticated.</p>
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<p>Release date: <strong>April 2025</strong></p> 
<p><a class="btn primary" target="_blank" onclick="_paq.push(['trackEvent','conversion','oots-tdd','button-view-chapter-2'])" href="https://ec.europa.eu/digital-building-blocks/sites/x/6hqlNQ"><span>View chapter</span><span class="sr-only">(opens in a new tab)</span><span class="ico-external"></span></a></p>
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<p><small>Chapter 3</small></p>
<h3 class="h5">Common Services</h3>
<p>This chapter looks at the Common Services necessary for the proper execution of the evidence exchange process. You can also learn more about APIs supporting these common services.</p>
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<p>Release date: <strong>April 2025</strong></p> 
<p><a class="btn primary" target="_blank" onclick="_paq.push(['trackEvent','conversion','oots-tdd','button-view-chapter-3'])" href="https://ec.europa.eu/digital-building-blocks/sites/x/DRulNQ"><span>View chapter</span><span class="sr-only">(opens in a new tab)</span><span class="ico-external"></span></a></p>
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<p><small>Chapter 4</small></p>
<h3 class="h5">Evidence Exchange</h3>
<p>This chapter introduces evidence exchange in the Once-Only Technical System, which is based on bilateral data exchange between relevant authorities.</p>
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<p>Release date: <strong>April 2025</strong></p>
<p><a class="btn primary" target="_blank" onclick="_paq.push(['trackEvent','conversion','oots-tdd','button-view-chapter-4'])" href="https://ec.europa.eu/digital-building-blocks/sites/x/7RulNQ"><span>View chapter</span><span class="sr-only">(opens in a new tab)</span><span class="ico-external"></span></a></p>
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<p><small>Chapter 5</small></p>
<h3 class="h5">Data Models</h3>
<p>This chapter looks at the data models that streamline the exchange of evidences, together with a methodology for defining new data models and code lists.</p>
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<p>Release date: <strong>April 2025</strong></p>
<p><a class="btn primary" target="_blank" onclick="_paq.push(['trackEvent','conversion','oots-tdd','button-view-chapter-5'])" href="https://ec.europa.eu/digital-building-blocks/sites/x/hRylNQ"><span>View chapter</span><span class="sr-only">(opens in a new tab)</span><span class="ico-external"></span></a></p>
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<p><small>Chapter 6</small></p>
<h3 class="h5">OOTS Guidance & UX <br>Recommendations</h3>
<p>This chapter provides an overview of how the Once-Only Technical System works as part of the wider Single Digital Gateway and looks at the first set of UX recommendations.</p>
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<p>Release date: <strong>April 2025</strong></p> 
<p><a class="btn primary" target="_blank" onclick="_paq.push(['trackEvent','conversion','oots-tdd','button-view-chapter-6'])" href="https://ec.europa.eu/digital-building-blocks/sites/x/kxylNQ"><span>View chapter</span><span class="sr-only">(opens in a new tab)</span><span class="ico-external"></span></a></p>
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<a class="btn ghost" target="_blank" href="https://ec.europa.eu/digital-building-blocks/sites/x/oBqlNQ"><span>Changelog</span><span class="sr-only">(opens in a new tab)</span><span class="ico-external"></span></a>
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