9F5F

Data Storage Slot Availability is a single byte the card returns stating whether it currently has room for a new data-storage write - the feature's own capacity signal, since a card's onboard storage for this Kernel 2 feature is necessarily finite and can fill up across repeated program participation over the card's lifetime. It's related to both the Requested Operator ID (9F5C) and the DSDOL (9F5B), both covered earlier, forming the sequence a terminal walks through to actually use the feature: identify the operator (9F5C), learn what data to supply (9F5B), and check whether the card can even accept it (9F5F) before attempting the write. A terminal that ignores 9F5F and attempts a data-storage write to a card reporting no available slot will have that write silently fail or be ignored by the card, which is worth checking explicitly rather than assuming every data-storage attempt succeeds just because the terminal supplied well-formed data. See EMV Contactless Book C-2.