Salesforce Campaigns are the best way for Sales Cloud users to manually create a segment of Leads and Contacts. Through Marketing Cloud Connect, Salesforce Campaigns can be used directly as an audience source in the Content Builder Send Flow, allowing quick selection and targeting without custom queries.
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Exact Extracts from Salesforce Documentation:
Source: Salesforce Help - Marketing Cloud Connect and Salesforce Campaigns
# "Salesforce Campaigns can be used as an audience for sending emails from Marketing Cloud. Campaign Members (Leads and Contacts) can be targeted directly." Source: Salesforce Help - Send Flow and Salesforce Data
# "When sending an email using Content Builder's Send Flow, users can select Salesforce Campaigns as the audience source if Marketing Cloud Connect is enabled."