For most servo drives, set DRIVE_MODE in the MC_CONFIG. On EtherCAT startup, the Trio Master will set up the drive for the selected mode and set the ATYPE automatically.

Some drives can change mode after EtherCAT startup. In the PDO you will see the Mode of Operation object 0x6060 is mapped to the cyclic data. In that case you can set ATYPE any time and change mode. So long as the drive allows it.

The image below is an example of how a drive can have multiple modes that can be switched by ATYPE: