A sub-pattern of 'uN' (N = 1..4) indicates that the serialized buffer will contain (at the current offset) an unsigned (big-endian) integer across 'N' bytes. The 'u' pattern consumes one field in the cmd struct.
This value will be copied to the working field in the target structure. All 'u' pattern values are casted to unsigned (4 byte) int. The four 'u' sub-patterns are 'u1' (a single unsigned char), 'u2' (an unsigned, big-endian short), 'u3' (an unsigned big-endian, 3-byte integer), and the described 'u4'. Again, these are promoted to 4-byte integers when copied by value into the target struct.