To validate that a parent-level data extension can be accessed by a child business unit, the developer should ensure that the data extension is in the Shared Data Extensions folder and the query includes the ENT. prefix (A). This setup allows shared data extensions to be accessible across different business units.
References:
Salesforce Marketing Cloud Shared Data Extensions
Query Activities with Shared Data Extensions

To append the subscriber's zip code to a link and ensure that the clicks are tracked, you should use the Lookup function to retrieve the zip code from the data extension and the RedirectTo function to create the link with tracking enabled.
* Lookup: Retrieves the zip code from the data extension.
* RedirectTo: Ensures that the link with the appended zip code is tracked.
* Example:
ampscript
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%%[ SET @zipcode = Lookup("DataExtensionName", "zipcode", "SubscriberKey", _subscriberkey) SET
@link = Concat("http://example.com?zip=", @zipcode) ]%% <a href="%%=RedirectTo(@link)=%%">Click Here</a> Salesforce AMPscript Lookup Function Salesforce AMPscript RedirectTo Function

The IIF function in AMPscript is a shorthand way to perform conditional logic within an inline expression. It evaluates a condition and returns one of two values based on whether the condition is true or false.
* AMPscript IIF Example:
%%=IIF(@level = 'premier', 'You are a premier member!', 'Upgrade to premier now!')=%% Salesforce AMPscript IIF Function

When using the REST API to send emails to customers after a purchase, the developer should make a token API call and re-use the token until the token expires (D). This approach optimizes API usage by reducing the number of token requests, ensuring that the token is reused until it naturally expires, after which a new token can be requested.
References:
Salesforce Marketing Cloud API Authentication
Token Management Best Practices

A 401 Unauthorized HTTP status message indicates that the request was not authenticated properly. This error commonly occurs when the access token is missing, invalid, or expired.
* Access Token Issues: The most common cause of a 401 error in REST API requests is related to the access token. The request must include a valid access token in the authorization header.
* Example Authorization Header: Authorization: Bearer YOUR_ACCESS_TOKEN Salesforce API Access Tokens