Oftentimes, JGroups sends messages which have only headers but no payload. These messages (e.g. heartbeats, stability/stable messages) could use another message to piggy back on. Or - if no regular message is sent within a certain timeout - all accumulated piggy back messages could be merged into one.
The advantage is that we send less traffic.
Implementation could be via a flag PIGGYBACK (same as OOB) in Message, and a protocol PIGGYBACK. The protocol accumulates all messages with the flag PIGGYBACK set, and attaches them to a regular message, or - when a timeout elapses - merges as many of the piggy back messages as possible and sends them as one message.

Note that this is not the same as the old PIGGYBACK protocol which used to be in JGroups a long time ago.