AWG stands for American Wire Gauge. We don't use this as a measurement in the UK, but we have to for vaping products because many of the references start out from America.
Here is a list of the actual AWG with their comparative mm equivalents.
38 AWG - 0.1MM//37 AWG - 0.11MM//36 AWG - 0.13MM
35 AWG - 0.14MM//34 AWG - 0.16MM//33 AWG - 0.18MM
32 AWG - 0.2MM//31 AWG - 0.23MM//30 AWG - 0.26MM
29 AWG -0.29MM//28 AWG - 0.32MM//27 AWG - 0.36MM
26 AWG - 0.41MM//25 AWG - 0.46MM//24 AWG - 0.51MM
23 AWG - 0.57MM//22 AWG - 0.64MM//21 AWG - 0.72MM
20 AWG - 0.81MM//19 AWG - 0.91MM//18 AWG - 1.02MM
17 AWG - 1.15MM//16 AWG - 1.29MM//15 AWG - 1.45MM
In other words mm is a far more precise measurement. All sizes that are between AWG measurements will be rounded to their nearest AWG equivalent. For example 0.3mm is between 29 AWG and 28 AWG. We have rounded it to the nearest AWG measurement. We have put +- to indicate this.
We are always very happy to help. Please email us if you require any more information on any of our products.