How must be the downward communication from the superior to the subordinate?

There formal communication in between subordinate and superior, here the superior sends directions or instruction to his subordinates is downward communication.

Downward communication through the superior to the subordinate should be, like: complete in all respects, issued within a clear language, rational, timely, and capable of implementation, while writing and explained clearly as to its purpose as well as brief and to the point.