Technically, the MP3 format has an upper ceiling, usually capped around 320 Kbps. When you see "640 Kbps Mp3" being discussed or searched for, it usually refers to one of two scenarios:

While certain encoders like can technically produce a "non-standard" 640 Kbps MP3, these files are often unplayable on most standard devices and provide no audible benefit over standard high-quality formats. Understanding Audio Bitrates for Hindi Music

One term that has recently gained traction in premium audio circles is .