# 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.

The Design & Code editor guide includes the [full list of reserved substitution tags][subtags].

[subtags]: /docs/sendgrid/ui/sending-email/editor#reserved-substitution-tags

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

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