Define UChar32 as a type for single Unicode code points. UChar32 is a signed 32-bit integer (same as int32_t).
The Unicode code point range is 0..0x10ffff. All other values (negative or >=0x110000) are illegal as Unicode code points. They may be used as sentinel values to indicate "done", "error" or similar non-code point conditions.
Before ICU 2.4 (Jitterbug 2146), UChar32 was defined to be wchar_t if that is 32 bits wide (wchar_t may be signed or unsigned) or else to be uint32_t. That is, the definition of UChar32 was platform-dependent.