A customer indicates their point-of-sale system can be configured to upload a file every fifteen...

Each File Drop Automation must monitor a unique directory, and that directory cannot be shared across multiple File Drop Automations (B).

The file trigger directory must be inside the import folder structure for Enhanced FTP (C).

Using an external FTP site is not supported for File Drop Automations — it must be Salesforce's Enhanced FTP. The limitation on the number of triggers is not five; it's based on system and licensing limits.

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Exact Extracts from Salesforce Documentation:

Source: Salesforce Help – File Drop Automation Requirements

→ "Each File Drop Automation must have its own unique directory. The directory must reside within the Import directory."

Source: Salesforce Help – Enhanced FTP File Drop Automations

→ "File Drop Automations require files to be dropped into a directory under the Import folder on Marketing Cloud’s Enhanced FTP."