Indicate the chunk within the field with "C=xxx n1 to n2" where xxx is "word", "char", or "item", n1 is the starting chunk and n2 is the ending chunk. If n2 is missing, only one chunk will be used. If you do not indicate the chunk, the whole field will be used.
For example, if you want to use characters 3 to 7 of the field as the key, use: c=char 3 to 7