There are a number of ways defenders and markers can help each other by talking. Already we have seen the use of "Up", "Up long" and "Break" calls by the marker to let the defender know the disc is in the air or the force has been broken.
A common call from the defender to the marker is "No out" or "No break". This is used when the receiver is open on the break side and tells the marker to make doubly sure that there is no break of force, even if the open side throw is much easier. The other one that is often used is the "Strike" call, which asks the marker to switch the force for a second or two to shut down an open side cut.
This is most important when switching and poaching, as well as in zone and clam defences. See those sections for more details.
The sideline can see which side cutters are coming in from and where the greatest points of danger are on the field. They are therefore in the best position to communicate with the marker and tell them what to do. This may include telling them which way to force, not to let the thrower break, to switch the force, to get their hands up or down, etc. It is better to let the defender call strikes since they are better able to judge when they have been burnt!
Often it is hard for the marker to make "Up" and "Break" calls loudly and quickly. This can be because they are out of breath or because they are more easily fooled by fakes. The players who are standing on the sideline can help the other players by making these calls as well. Also, the sideline has a better perspective on the game than the players on the field, so they can call to specific players, for example if they can make a play on the disc, but have their back to it.
This is a very important part of communication, and is one reason the sideline is often called the eighth man.