This documentation is no longer maintained and exists for historical purposes. The current documentation is located at http://suricata.readthedocs.io/.
A packet consists of raw data. HTTP and reassembly make a copy of those kinds of packets data. They erase anomalous content, combine packets etcetera. What remains is a called the 'normalized buffer'.
Because the data is being normalized, it is not what it used to be; it is an interpretation.
Normalized buffers are: all HTTP-keywords, reassembled streams, TLS-, SSL-, SSH-, FTP- and dcerpc-buffers.