By the conditions of time and place. When the messengers to communicate necessary tools to associate, communications such as mail delivery. Speaking on the telephone.
The terms of the time and place so when a messenger to communicate we need the tools to link to. Communication, such as sending letters to each other. Speaking by phone
The conditions of time and place, so when the sender to communicate it requires a tool to link can communicate each other. For example, send a message, speaking on the phone.