I've been wondering which field should I use when collecting user purchases, views, etc. There are a couple of different resources in Salesforce about this and both of them tell differently.
First one is under Email Recommendations -> Implementation, inside marketing cloud itself:
And it says that Subscriber Key should be used.
The other one is from Salesforce Docs (https://help.salesforce.com/s/articleView?id=sf.mc_ctc_set_user_info.htm&type=5):
This one says that Subscriber ID should be used.
As far as I understand it, these two are completely different. I use Contact Id from Salesforce as my SubscriberKey and I think Subscriber Id is generated inside marketing cloud itself.
The JSON itself has a field that's named "email" - the name suggests that email should be used.