You can optionally associate a numeric identifier with a class. The Ice run time substitutes this value, known as a compact type ID, in place of its equivalent string type ID during marshaling to conserve space. The compact type ID follows immediately after the class name, enclosed in parentheses:
In this example, the Ice run time marshals the value
4 instead of its string equivalent
"::M::CompactExample". The specified value must be a non-negative integer that is unique within the translation unit.
Using values less than 255 produces the most efficient encoding.