 Tax Application: When product added to invoice, selected tax rates automatically applied to line item for calculation. Purpose: Staff-only notes not visible to customers Textarea input (3 rows display) Plain text only Never shown on invoices, emails, or customer-facing pages Useful for internal documentation Data Source: _product_internal_notes post meta (string) Example Use Cases: Supplier information or part numbers Cost basis for margin calculation Product sourcing notes Special handling instructions Deprecation notices or workflow reminders