This topic will be part of the Application Guideline in future. Some general recommendations:
Put all devices of one EtherCAT string into a Sync Unit.
Additionally you can also put every device type (especially non-Beckhoff devices) into an own Sync Unit.
Fur more detailled analysis in case of bus problems, put every device in its own Sync Unit.
Safety devices (Beckhoff EL6910, PNOZmulti, etc.) should be in an own Sync Unit.
Bus couplers (EL6692, EL6695) should be in an own Sync Unit.
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