Since we are talking about a code of conduct manual here, we are presumably referring to guidelines that fall under the category of conduct, not those that have mainly to do with style. Therefore, these guidelines will not relate to the style of the writing to any great degree.
The major guideline in this regard would be that all employees must treat one another with respect in their written communications. This would be true regardless of whether the person writing is higher, lower, or at the same rank in the firm’s hierarchy as the person receiving the communication. Specifically, then, the guidelines would call for no profane or obscene language. They would call for writers to refrain from using personal attacks. (In other words, the guidelines would say that a person should write “this proposal does not seem likely to be feasible” rather than “you have produced a stupid proposal.”) They would not prevent a superior from giving frank criticism of a proposal or an action, but the criticism should be done in a polite and respectful way.
Proper communications in a business setting must be respectful. Communications that fail to show respect damage employee morale and relationships and thereby harm the company.