To manually create a segment of both lead and contact records that can be targeted through the Send Flow in Content Builder, Northern Trail Outfitters should use a Salesforce Campaign. A Salesforce Campaign is an object in Sales Cloud that allows marketers to group leads or contacts for a specific marketing initiative, such as an email campaign. A Salesforce Campaign can be used as a target audience in Marketing Cloud using the Send Flow in Content Builder. References:
https://help.salesforce.com/s/articleView?id=sf.campaigns_overview.htm&type=5https://help.salesforce.com/s/a