Cumulative is used because the interesting values are defined over a cumulative range of time (+E8). This term is more precise than the existing term (-E3, +E9). Instantaneous aggregation may be considered to be a degenerate case of cumulative aggregation where the partition is per chronon and the associated interval is that chronon.
One example of cumulative aggregation would be find the total number of employees who had worked at some point for a company. To compute this value at the end of each calendar year, then, for each year, define a valid-time element which is valid from the beginning of time up to the end of that year. For each valid-time element, find all tuples which overlap that element, and finally, count the number of tuples in each set.