Military Contact

Culture level is a Critical Success Factor.
Tier 1 Suppliers are operating Just-in-Time and their quality is critical to your quality requirements.
If you view my YouTube video "Cost of Quality vs. Sigma" it shows a sweet spot of 4.25 Sigma. This allows 0.25 Sigma process degradation that still leads to the optimum quality performance level of 4.00 Sigma. Toyota's goal is 6+ Sigma.
Tier 2 Suppliers are critical to you quality standards but don't operate Just-in-Time.
Tier 3 Suppliers include all other Suppliers.