Once you hit 6gb, you stop paying till you hit 15gb. After that, you're rate limited, or you can call up Fi support and have them put you at full speed and charge the usual $10/gb
That cap's been there for a while. IIRC, it was implemented some months after T-Mobile started the "race to unlimited data"... was that the winter before last? Speaking of the U.S. marketplace. I never read an official explanation, but my assumption/understanding was that this was Fi's response to remain competitive with other carriers' "unlimited" plans.
What concerns me is the throttling at/after 15 GB. Other carriers have this, too, but it is supposedly "dependent on local load/conditions", and comments have led me to understand that, for some if not many people, they don't encounter the slowdown much in practice.
Whereas, being an MVNO and also perhaps having better software, Fi might be more strict and aggressive with the throttling? I don't know.