1. (I) A tree-structured (loop-free) topology of relationships between CAs and the entities to whom the CAs issue public-key certificates. (See: hierarchical PKI, hierarchy management.)
2. (I) /PEM/ A certification hierarchy for PEM has three levels of CAs [R1422]:
- The highest level is the "Internet Policy Registration Authority".
- A CA at the second-highest level is a "policy certification authority".
- A CA at the third-highest level is a "certification authority".