On the lead request page, the "opt-in" field indicates whether the lead authorises you to use their personal information for commercial purposes.
You can therefore use this file when sending newsletters or other commercial offers.
Marketing definition of opt-in:
Opt-in is the principle whereby an individual must give their consent before they can be targeted by a direct sales campaign conducted by an automated marketing platform. The opt-in system is a legal obligation specified in the LEN (Digital Economy Law) and in the application recommendations issued by the CNIL (French Data Protection Commission)