There are many tools with which to use for communication.
What you need to communicate will depend on what you use to send it.
Assumptions: Factors that will be considered true, real, or certain for planning purposes.
(Assumptions generally involve a degree of risk and may be identified here or in the risk identification process).
Finding the responsible team member: Project manager can do this on his/her own, but involving team members or executives in the communication ensures high level of commitment to the project.