Signed packets are a special type of packets that are guaranteed to arrive.
Normally when using UDP multiplayer, packets are sent very fast on the cost of reliability.
Signed packets give GMnet ENGINE the option to have packets that are sent with the reliability of TCP. These are used with the sync types SMART and IMPORTANT and some other internal operations.
You can also use them. Either by using the SMART and IMPORTANT sync types or by using them with your own packets. For how see “Extending the Engine / Advanced Use”.