Each dragon who wields at least one element has a certain level of experience with this element. Such experience is called skill level.
This skill level applies to someone who can barely use their element(s), or has no control over it. Usually this skill level only applies to little kids, as they will grow to control their elements even with little training.
Examples: Validor Starwing
Basic level. The user has control of their element, but it is relatively weak.
Examples: Tori Jewelhorn
The most common skill level. These dragons can control their element just fine, and it is capable in combat.
Examples: Electroy Thunderfang
To reach this level, one usually has to go through training. The master skill level opens up new types of element manipulations, including the elemental fury.
Examples: Burner Phoenixwing
Dragons, usually trained by the Guardians, can achieve this skill level. They know everything known about their element(s), each secret attack, ability, everything.
Most of time a dragon of a guardian level is also a guardian.
Examples (Darkers): All Alpha Darkers