Prepending a u to the type indicates that ordered comparisons should be unsigned. Numeric values may be preceded by a character indicating the operation to be performed. For string values, the byte string from the file must match the specified byte string. The length used for matching is that of the string argument in the magic file. The message to be printed if the comparison succeeds.
Some file formats contain additional information which is to be printed along with the file type.
The next line at level n terminates this. That means that the number after the parenthesis is used as an offset in the file. The value at that offset is read, and is used again as an offset in the file. The value of x is used as an offset in the file. The capitalized types interpret the number as a big endian value, whereas the small letter versions interpet the number as a little endian value.