It’s when multiple references have the same combinaison “Author/Year” Zotero automatically applies a suffix to distinguish the references.
For example:
OECD (2011). Strategic Transport Infrastructure Needs to 2030: Main Findings OECD.
OECD (2011). Environmental Impacts of International Shipping: The Role of Ports. OECD Publishing.
Should appear like this:
OECD (2011a). Strategic Transport Infrastructure Needs to 2030: Main Findings OECD.
OECD (2011b). Environmental Impacts of International Shipping: The Role of Ports. OECD Publishing.