The difference between a flat drill (ADF series) and an end mill for counterboring (ZDS series) is as follows:

ADF can be machined to twice the depth of the tool diameter. Oversize can also be processed stably.

ZDS can be machined to a depth of 1 times the diameter of the tool. It is a specification that makes it easy to achieve the flatness of the bottom of the hole.

Better processing can be performed by using them properly.