To use datasets together in a query, they must also have at least one key in common.
Keys are the anonymised information InfoSum uses to identify records in separate datasets which relate to the same individual. A key might be a single piece of data, or a combination of data items which together uniquely identify a person.
Once you've configured your categorisations and transformations, your Bunker generates keys for each row. Keys are anonymised version of the row's PII, used solely to match up records in your and your partner's Bunkers.
Two Bunkers with the same PII will generate the same key, so the two Bunkers can use their keys to identify records which relate to the same person. But it isn't possible to use a key to recover the original information. So, even in the unlikely event that a key was compromised, this wouldn't reveal the PII.
You can check whether datasets share a common key on the Connections tab of Dataset Manager. Here, you can also see the keys that appear in both datasets, the intersection, and the number of times a key is duplicated, i.e. the same email appearing in two rows.