How can I customize tax code labels?

You can rename MST (Main Service Tax) and MET (Main Expense Tax) on the Custom Labels screen under Settings > Display & Formatting > Company. These labels name the tax amount fields when you create or view an invoice in CORE, but they do not change the tax name that prints on invoices. That name comes from the Tax Rates catalog. To change it, edit the tax's Name in Settings > Billing & Invoices > Tax Rates.