Similar to +Omer G. Joel, I read the skills as follows:

Skill level 0: basic familiarity, can do ordinary activities involving the skill. If you have a standard driver's license, you have Drive/Wheeled-Vehicle 0.

Skill level 1: You have more familiarity, and can handle some exceptional situations. Drive/Wheeled-vehicle 1 would be someone who can drive a Land Rover off-road without flipping it over. Or, you have a higher level of familiarity with the relevant laws and are thereby permitted to drive for hire ('cabbie' or 'hack' license). Or, you can drive in both the US (right side of road) and UK (left side of road) with equal facility

Skill level 2: You have a great deal of familiarity, and can handle many exceptional situations. Police trained for chase/evade driving have Drive/wheeled-vehicle 2

Skill level 3 and above represent increasing ability to handle exceptional situations, speeds, or terrains. If you can race cars professionally, you have at least Drive/Wheeled-vehicle 3

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Rank 0 is "certified" - you know the basics, but cannot hold a job with that skill yet. For example, military basic firearm training (Gun Combat-0); first-aid training (Medicine-0).

Rank 1 is "employable" - you are skilled enough to actually get paid to use that skill. For example, a skilled mercenary (Gun Combat-1); a paramedic (Medicine-1).

Rank 2 is a "profession" - in civilized societies you would be able to get a license to practice a profession with that, or at least get paid better than an ordinary worker. For example, a marksman (Gun Combat-2); a nurse or medical intern (Medicine-2).

Rank 3 is a "skilled professional" - you can definitely get a license if needed, and you earn professional-level pay. For example, a sniper (Gun Combat-3); a skilled physician (Medicine-3).

Rank 4 and above denote even greater expertise.

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