The 20 x 6mm wick tab provides an added level of safety as compared to the 15 x 3.8 tab. Its longer neck snuffs the flame sooner reducing the chance of containers getting hot when the candle is nearing the bottom.
CandleScience wick tabs are made of thick steel coated with tin to resist corrosion by essential oils and fragrance. Wick tabs are also available in the smaller 15x3.8 size.
The large hole size is a convenient choice that can be used on any size wick.