By passing a format specifier to scale.tickFormat, you create a number format with precision appropriate to the scale’s tick values. When a SI-prefix format type is used (type s), the scale also computes a consistent SI-prefix for all ticks, as shown on the left; this feature is new in 3.4.4. In contrast, when d3.format is used directly as shown in the middle, the SI prefix can precision can vary for each tick. In 4.0, you can also create a consistent SI-prefix formatter manually using d3.formatPrefix.