What it is
Available Offline Spending Amount (AOSA) is the remaining amount the cardholder may spend offline, computed by the card from its offline accumulator and limits. Contactless kernels can return it so the reader (or a companion app) can display "available to spend offline" to the cardholder.
Where it appears
Returned by the card during contactless processing.
Specification reference
Defined in: EMVCo Contactless Book C-2 (Kernel 2 / Mastercard); also Book C-3
Meaning by contactless kernel
Kernel 3· Visa
Supported where offline data storage / balance display is enabled.