---
"@context": https://schema.org
"@type": TechArticle
"@id": https://www.twilio.com/docs/glossary/system-fields#article
headline: Reserved substitution tags
description: "*Configuration*. Placeholder labels that get replaced with personalized content in an email campaign."
url: https://www.twilio.com/docs/glossary/system-fields
inLanguage: en
dateModified: 2026-05-06T13:22:30.000Z
author:
  "@type": Organization
  name: Twilio Developer Education Team
publisher:
  "@type": Organization
  name: Twilio
---

# Reserved substitution tags

*Configuration*. Placeholder labels that get replaced with personalized content in an email campaign.

Though an email campaign can use placeholders based on any [custom][] or [reserved][] field, Twilio makes these tags available to any email campaign.

[custom]: /docs/glossary/custom-fields

[reserved]: /docs/glossary/reserved-fields