Proof of ID and earnings
The sensitive nature of the financial services industry requires Lydia to know its users. The Banque de France's ACPR (Autorité de Contrôle Prudentiel et de Résolution) makes this very clear.
To make creating an account from your phone easier, and since there are few risks, the identification requirements for each user are initially minimal. You just have to declare your identity when signing up.
To use the app beyond a certain limit or after a certain amount of time, you'll have to prove your identity with recognised documentation.
When you verify your account to have these rights, Lydia may ask you to provide supplementary documentation, such as a tax assessment or a payslip.
The precaution taken is proportional to how much you use the app. If you were to receive, for example, over £4,000 or €4,000 on one of your Lydia accounts, you may have to prove where the money has come from. If this is a regular sum coming into your account, you won't have to justify it each time - once will suffice.