Using negation can be useful if you would like to do some generic processing but exclude some specific events. You can use the discard action in conjunction with that. A sample would be:
. /var/log/allmsgs-including-informational.log
:msg, contains, "informational" ~
. /var/log/allmsgs-but-informational.log
Do not overlook the red tilde in line 2! In this sample, all messages are written to the file allmsgs-including-informational.log. Then, all messages containing the string "informational" are discarded. That means the config file lines below the "discard line" (number 2 in our sample) will not be applied to this message. Then, all remaining lines will also be written to the file allmsgs-but-informational.log.