To understand HIGHESTPARENT and LOWESTPARENT, take an axis that has three fields: Year, Quarter and Month. The HIGHESTPARENT is Year, while the lowest parent is Quarter. For example, a visual calculation that is defined as RUNNINGSUM([Sales Amount], HIGHESTPARENT) or RUNNINGSUM([Sales Amount], 1) returns a running sum of Sales Amount that starts from 0 for every year. A visual calculation defined as RUNNINGSUM([Sales Amount], LOWESTPARENT) or RUNNINGSUM([Sales Amount], 2) returns a running sum of Sales Amount that starts from 0 for every Quarter. Lastly, a visual calculation that is defined as RUNNINGSUM([Sales Amount]) does not reset, and will continue adding the Sales Amount value for each month to the previous values, without restarting.