Addenda incorporate changes and improvements to the LEED rating systems and reference guides to help clarify, correct, interpret or provide alternative language. Addenda = LEED Interpretations + Corrections.
LEED Interpretations, or LIs, are precedent-setting official answers to formal technical questions about implementing LEED that can be used by all projects. Project teams are required to follow Addenda issued before their project's LEED registration date. Addenda issued after a project's registration date are optional—but USGBC recommends their use (and they often make LEED more clear and easier to follow, so there are multiple reasons for using them).