BB Gauge Refill (commonly referred to as “infinite SBB” or “iSBB”) is an effect that refills the unit’s BB gauge to full.
Most currently released units with BB Gauge Refill and SBB obtainable fill their SBB gauges to full, with the exception of Skilled Lance Noah who only fills his BB gauge to full. Another exception is Endless Cycle Limilnate, whose SP option “Adds enormously boosts own BB gauge effect to BB/SBB” fills 20 BC, which is just enough for his BB. Avalanche Jack only fills 30% of his gauge when using SBB.
Despite the nature of this effect filling the BB gauge to full, it can still be affected by negative values of a BC Efficacy Reduction debuff.