The X1 Carbon/Extreme series used to be the only products labeled Gen x instead of a 2/3 digit code, e.g. T490. Starting this year they rebranded all ThinkPad products to use generation numbers (presumably because rolling from something like T490 to the next number would be a little difficult, kind of like when GPUs hit the 9xxx branding mark), so X390 -> X13 gen 1, P53 -> P15 gen 1 etc.