Synch Selector Switch - Auto Transfer Scheme

For the LV switchgear ATS (Auto Transfer Scheme), have you used a synch selector switch?

I am familiar with using a trip selector switch (TSS), where the TSS trips the selected breaker after momentary parallel operation.

How does the synch selector switch work in this scheme?

Are both the synch selector switch and the trip selector switch used together, or is only one of them required?