Yesware will automatically sync the meeting with the Contact or Lead in Salesforce associated with the first email address added as a guest to the calendar invite.
It will sync it to Salesforce as an Event in the Open Events until the date and time has passed, and it will then live under the Activity History.
If no match is found and you don’t manually enter a record, then it will sync the event to Salesforce with no associated Contact/Lead and Account/Opportunity.
All Events that Yesware logs into Salesforce using Calendar Sync will have "[event synced]" added to the Description field of the event in Salesforce. This notation is only added to the Salesforce event and is not included in your O365 Calendar event.