Background on what these are and how to set them up: Using Unique IDs when you import people from a CSV.
What happens if I import people without Unique IDs?
Yapp falls back to guessing who's who, and how it guesses depends on the option you pick at the end of the import:
Replace all existing data — Yapp compares first name, last name, title, and company. All four have to match exactly for it to treat the row as someone it already knows. Rows where anything has changed are treated as new people, and the originals come off the page.
Add entries to existing data — Yapp doesn't try to match at all. Every row becomes a new person.
So a second upload either duplicates your whole list, or quietly loses track of anyone whose details have changed since the first one. Correcting a typo in someone's job title is enough to do it.
Does it matter whether I choose Add or Replace?
Not for matching — Add entries to existing data and Replace all existing data both match on Unique ID, and both update the people they match.
The difference is what happens to people already on the page who aren't in your file. Replace takes them off the page; Add leaves them there. Either way they stay in your app, so you can add them back later.
Can I change a person's Unique ID later?
Treat them as permanent. What happens if you change one depends on the option you pick, and neither outcome is worth relying on:
With Add entries to existing data, Yapp sees an ID it doesn't recognize and creates a second copy of that person.
With Replace all existing data, if the person's name, title, and company are otherwise unchanged, Yapp may recognize them that way and simply move them to the new ID.
If you want to use your own IDs rather than the ones Yapp assigned, decide on them once, before your first upload.
Can two people have the same Unique ID?
They shouldn't. Yapp checks the file you upload and flags an ID that appears twice in it, but it doesn't check against the people already in your app. If two people do end up sharing an ID, an import matching that ID updates one of them — and not necessarily the one you meant.
Where do I find a person's Unique ID?
Unique IDs don't appear anywhere in the editor. Export your People page as a CSV and it's the first column, already filled in for everyone.