The strength of bleach depends on the concentration (%) of the peroxide. The bleaching effect will be stronger for a higher concentration which will further brighten the hair, but this will also lead to hair damage. A lower concentration will be gentler to the hair, but the bleaching effect will be weaker and this will not effectively brighten the hair.
In general, the concentration of a permanent hair color developer is between 6% and 9%, while it is 2% to 3% for a demi-permanent hair color.