Data Provenance
A documented record of where data came from, how it was collected, and what may be done with it.
Provenance answers the questions that arrive with an audit or a lawsuit: whose data is this, on what basis was it collected, and are we allowed to train on it. Retrofitting it is painful and sometimes impossible. It also underpins reproducibility and poisoning defence.
In practice: Being able to show that no scraped personal data entered the training set.