Explanation
To integrate a new dataset with pre-existing contacts, Northern Trail Outfitters should create a new data extension and link it to the other data extension. A data extension is a table that stores data in Marketing Cloud, such as contact information or purchase history. Data extensions can be linked by common fields or attributes using Contact Builder or SQL queries. References:
https://help.salesforce.com/s/articleView?id=sf.mc_es_data_extensions.htm&type=5

Explanation/Reference:

Explanation
A parent business unit is the top-level business unit in an account hierarchy that can share data, content, and settings with its child business units. A child business unit is a lower-level business unit that inherits data, content, and settings from its parent business unit but can also have its own data, content, and settings. By creating a parent business unit for the corporate office and a child business unit for each franchise owner, the customer can achieve data and content sharing as well as customization and segmentation for each local store.
References: https://help.salesforce.com/articleView?id=sf.mc_overview_business_units.htm&type=5

Journey Data is the data that is captured at the time of entry into the journey - it's a snapshot of the specific data related to that specific interaction (such as the specific tent purchased).
Because customers can have multiple tent purchases, you need to make decisions based on the data specific to that journey entry (not the general contact record), which is Journey Data.
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Exact Extracts from Salesforce Documentation:
Source: Salesforce Help - Journey Builder Decision Splits
# "Use Journey Data to evaluate information captured at the time a contact entered the journey, ideal for handling multiple purchases or time-specific entries." Source: Salesforce Help - Journey Data vs Contact Data
# "Journey Data is dynamic and tied to a specific journey instance. Use Journey Data when evaluating data related to a specific event or purchase."
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