In this image, skill icon is denoted by the number 8
"Skill icons" (スキルアイコン sukiru aikon) are common abilities that nearly all units have based on their grade. Each icon shows which ability the unit has.
There are currently four types of skill icons: 
Boost, 
Intercept, 
Twin Drive!!, and 
Triple Drive!!!.
Some card abilities can cause units to gain or lose skill icons. As skill icons are not inherently tied to a unit's grade, abilities that alter the grade of a unit will not affect its skill icon.
Boost
Japanese:「ブースト」(Būsuto)
- If your unit attacks in a battle phase, you can [Rest] a unit with "Boost" in the back circle of the same column as the attacking unit to have it boost the attacking unit. The power of the attacking unit increases by the power of the boosting unit.
- If your vanguard is in Legion, you choose one of the two vanguards to be boosted.
- The power of the boosting unit is continuously added to the attacking unit for the duration of the battle. If the boosting unit gains power, the gained power is added to the attacking unit. If the boosting unit ceases to exist, the boost power is lost.
- "Boost" only lasts until end of a battle.
- Currently only grade 0 and 1 units have "Boost".
- The exception to this is Takugal (grade 3) and Eternal Wing (grade 4), both of which are illegal.
Intercept
Japanese:「インターセプト」(Intāseputo)
- During your opponent’s guard step, if your unit with "Intercept" is in a front rear-guard circle, is not being attacked, and another of your units is being attacked, you can move the card with "Intercept" to your guardian circle.
- This is considered moving the unit from rear-guard circle to guardian circle (as opposed to placing it on the guardian circle), so "When this unit is placed on (GC)..." abilities do not activate.
- The units become treated as guardians in the guardian circle, and will retire once the battle ends regardless of whether or not the attack hits.
- Currently only grade 2 units have "Intercept".
Twin Drive!!
Japanese:「ツインドライブ!!」(Tsuin Doraibu!!)
- A Drive ability. "Twin Drive!!" gives the unit with the ability +1 drive, allowing them to drive check two cards instead of one during the drive step.
- Currently only grade 3 units and grade 4 normal units have "Twin Drive!!".
Triple Drive!!!
Japanese:「トリプルドライブ!!!」(Toripuru Doraibu!!!)
- A Drive ability. "Triple Drive!!" gives the unit with the ability +2 drive, allowing them to drive check three cards instead of one during the drive step.
- Currently only grade 4 G units have "Triple Drive!!!".
Trivia
- G guardians are the only units to lack skill icons.
- In the anime, units with abilities that grant them extra [Shield] upon intercepting are called "Especial Intercept".
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