In the NIST Cybersecurity Framework (CSF), "tiers" represent the degree to which an organization's cybersecurity risk management practices exhibit the characteristics defined in the framework (such as risk awareness, repeatability, and adaptability). Tiers range from Partial (Tier 1) to Adaptive (Tier 4) and describe the organization's overall cybersecurity maturity or profile.
Reference: NIST CSF v1.1, Section 2.2 ("Framework Implementation Tiers"); ISA/IEC 62443-1-1:2007, Section 4.2.7.