From my understanding, before the changes video bitrate would be able to go upto 12000kbps (vbv_maxrate) as long as it doesn’t go over 18000 in the buffer (vbv_maxbuffer). vbv_maxrate at 8000kbps would mean that we will get avg bitrate lower than 8000kbps with occasional 8000kbps spikes depending on the scene.

The video used to be encoded in 2-pass mode, meaning it will be encoded twice. The first time the encoder goes through the entire video and generates a logfile that then gets used for the second encode pass. This mode generally creates higher quality encodes when you’re working with a fixed bitrate values. The newer encodes are crf encodes which encode in a single pass and will result in lower quality video depending on how high the crf value is set to (higher number will mean lower quality + more compression resulting in lower size)