The SYN Level-1 product, also called Synergy product, results from the synergistic processing of OLCI Level-1B FR and SLSTR Level-1B products themselves derived from OLCI and SLSTR measurements (all channels).
User Guides - Sentinel-3 Synergy - Resolutions - Sentinel Online
SENTINEL-3 Synergy Resolutions
Sentinel-3 synergy User Guide key resources