Function nvenc_rate_ceiling 

fn nvenc_rate_ceiling(codec: Codec) -> u32

The highest CBR target an NVENC session of codec opens at, its top level’s bitrate ceiling: H.264 6.2’s 1 Gbit/s, HEVC 6.2’s 800 Mbit/s at the High tier production sessions declare, and NVENC_AV1_RATE of AV1 6.3’s Main-tier 160 Mbit/s. No level admits a rate past it, so the driver would refuse the session, or the change, as an invalid level.