Windows certreq full response file
The Full Response has additional information over and above the P7B file with information such as the CMC response and some chaining information.  I'd go as far as to say it's not really relevant in day-to-day use.

You can look at them all with certutil.exe and make your own mind up though - simply pipe the outputs to a text file and read at your leisure. If the extra parameter to your command above is webapp.ful the following will allow you to read all of the files with a text editor:

certutil webapp.cer &gt; cer.txt
certutil webapp.p7b &gt; p7b.txt
certutil webapp.ful &gt; ful.txt