A Unique ID is a short label that tells Yapp "this is the same person as before."
It only matters if you import or export your People page as a CSV. Build your list by hand in the editor and you'll never see it. But if you upload a spreadsheet — especially if you expect to upload a corrected one later — this is the field that decides whether your next upload updates your list or duplicates it.
What a Unique ID does
When you import a CSV, Yapp reads the Unique ID in each row and looks for a person in your app who already has that ID.
Found a match → Yapp updates that person with the details in your row.
No match → Yapp adds a new person.
Leave the column out and Yapp has to guess instead, by comparing names and job details. A second upload then either duplicates your whole list or loses track of anyone whose details have changed. See What happens if I import people without Unique IDs?
Why it's worth filling in
You can re-import as often as you like. People lists change constantly — a title gets corrected, a photo arrives late, someone drops out. With Unique IDs in place you fix your spreadsheet, upload it again, and everyone already in your app is updated in place instead of duplicated.
Corrections stop being risky. Without Unique IDs, fixing a typo in someone's job title is enough to make Yapp treat them as a different person. With one, you can change anything about a person and Yapp still recognizes them.
You can attach people to sessions from a spreadsheet. Schedule and Multi-Track imports have a People IDs column. Put people's Unique IDs in it, separated by ||, and Yapp adds those people to the session's list. It's the only way to do this in bulk. See Uploading data from a CSV for how the schedule columns fit together.
One person, many pages. Unique IDs belong to your app, not to a single page — so someone on your People page who's also attached to three sessions stays one person, with one photo and one bio to keep up to date.
Adding Unique IDs to your file
Add a column headed Unique ID to your spreadsheet and put a value in every row. Yapp matches the column by its heading, so you won't have to map it by hand during the import.
Almost anything can go in the cell. What matters is:
Every value is different. If the same ID appears twice in your file, Yapp flags it before the import runs.
It never changes. The value is the link between your spreadsheet and the person in your app. Change it and Yapp sees a new person. Registration numbers, staff IDs, and email addresses all work well; job titles and session names don't.
No commas. Commas separate one ID from the next in the People IDs column on schedule imports, so an ID containing one can't be referenced from a session.
Once you've settled on your IDs, use the same ones every time.
Finding the IDs Yapp has already assigned
If you're updating a list that's already in your app, you don't need to invent IDs for those people. Yapp has already given everyone one, whether you supplied it or not — when someone is added without an ID, Yapp builds it from their name, so Jane Doe becomes jane-doe.
You won't find it in the editor. To see them, export your People page as a CSV: Unique ID is the first column, already filled in.
Editing that export and uploading it back is the safest way to make a change, because an import rewrites the details of everyone it matches. Read What a re-import does to a person's details before you upload a file with only some of the columns in it.