Rebirth: Building the Strongest Party with my Overpowered Harem. - Chapter 7: Flames of Echidna.

... as a series of tasks that needed to take place.

If an F rank wanted to increase to an E rank, they would need to complete 15 F-rank tasks in a row, with no misses or failures.

The same went for E ranks to D ranks.

In fact, there have been cases of B-rank parties suddenly getting all the way to S rank. This was due to the fact that there was also a system in place for double ranking.

You needed to complete 25 tasks of the upper rank to get a boost that high.