Many of the packet engines are very generic. Rules are generally more
limited.
A rule like 'alert tcp any any -> any 888 (flags:S; sid:1;)' would still
be inspected against every SYN packet in most cases (it depends a bit on
rule grouping though).
This extra match logic adds an additional check to these packet engines.
It can add a check based on alproto, source port and dest port. It uses
only one of these 3. Priority order is src port > alproto > dst port.
For the ports only 'single' ports are used at this time.