The so-called "communication agreement" is the time, way and signal meaning agreed by both parties, otherwise it is turning a blind eye.
Once there is a "communication protocol", any common phenomenon may become a tool to transmit information.
For example, a German spy was found in the former Soviet Union. He was monitored for a long time and had no contact with the outside world. Later, it was discovered that this person prayed at home every day, blocked the lights with the rhythm of body ups and downs, and sent messages to the other side of the river.
More often, spies who are lurking deep will use "dead letters" to transmit information to avoid being discovered because of leaks. The way to deliver a "dead letter" is that the receiver just waits, the spy puts the information in a public position, and the contact just checks it regularly, not knowing who delivered the letter.
It used to be very troublesome for spies to send information to the outside world. It took several days and dozens of days to transfer manpower. Using radio waves to transmit, hundreds of bytes are busy for a long time. There is also the danger of leaks in the middle. Now information technology is developed, and a memory chip as big as a bean can store dozens of grams of information. The struggle between spies and counterspies has entered a new era.