Co-syndication refers to sharing an invitation to invest in an SPV with LPs from another syndicate. If Syndicate X is running an SPV into Company ABC, and a GP from Syndicate Y shares the link to that deal with their LPs, that's referred to as co-syndication. Co-Syndication is only permitted when the GP from Syndicate Y is providing advisory services on the deal, including but not limited to due diligence and monitoring services, to Syndicate X.
Co-syndication is not permitted to be used as a way to compensate any individual for any fundraising activities.
Any LP from Syndicate Y that invests in the SPV organized by Syndicate X will be considered an AngelList Platform LP and attributed to AngelList.