What Is a Data Append?
TL;DR
A data append is the addition of one specific missing field to records you already hold, such as attaching a phone number to rows that carry only an email address. It is the narrow case of enrichment: one field, matched on an identifier that is already on the row.
How does a data append differ from enrichment?
By scope rather than by mechanism. Both match your rows against an external source; an append fills one named field and enrichment fills everything missing. The distinction is worth keeping because an append is billed and measured per field, so its match rate is the rate for that field alone.
The practical difference is what you can ask for. If a list is complete except for phone numbers, a phone append answers exactly that and nothing else, which costs less and cannot overwrite fields you were happy with.
What is commonly appended?
Phone numbers onto email-only rows, and email addresses onto rows carrying a name and a company. Postal address for direct mail, LinkedIn URL to give a record a stable public identifier, and single firmographic fields such as headcount band or industry code where a list is otherwise usable.
Appends are also used to refresh rather than to fill. Replacing a stale job title across a list is the same operation as adding a missing one, and it is the append most worth running on a list that has been sitting unused.
What decides whether an append succeeds?
The strength of the identifier you already hold. A work email address or a LinkedIn URL identifies one person, so match rates are high. A first name and a company name identify a set of people, so the match is either ambiguous or wrong, and a confidently wrong append is worse than an empty cell.
Coverage of the segment is the second factor. Appending a direct dial in a market where phone data is thin will return little regardless of how good your identifiers are, which is why an append is worth running on a sample before the whole list.
Frequently asked questions
Only after verification, and only with the same care as any cold contact. The recipient never gave you that address, so it is a cold contact regardless of how the rest of the record was collected, and in the EU and UK you still need a lawful basis for processing it.