It’s not really related to the launch date of the model. Also, are you sure you aren’t mistaking with Mercedes?
BMWs use the letter “X” for their utilitarian line, with the numbers basically denoting size (the X5 is a much bigger model than the X1, for example).
The no-letter ones are the typical sedans, with the very first digit denoting the model’s shape (a 7xx is more spacious than a 3xx), where bigger usually also means more luxurious features (the 8xx is usually expensive af, for example). The following digits is usually related to the engine block (a 330 has a less powerful engine than a 340).
The “M”s are just beefier versions of the sedan series (more powerful engines and upgraded brakes, suspensions, etc to match). A M3 is the beefier version of the 3xx, M5 the 5xx, etc.
The “Z”s are the sporty ones. Iirc, all of them are two-seaters and convertible.
And then there are the “i”, which are especially designed around electrical engines.
Mercedes has a similar scheme, but a lot more letters, sometimes multiple letters (like the SLS).
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