A website is a complex system requiring specialized work from roles like developers, designers, and systems administrators. Having a full picture of these requirements will ensure that an organization's IT team can coordinate properly with the agency's before or after the launch.
The technical environment can be specified by an organization's IT team if, for whatever reason, the environment is fixed or limited. However, the agency is usually in a better position to define the environment based on the finished website's needs. In either case, it's important to identify the following:
Depending on the size and background of the organization's IT department, it may be entirely up to the agency to determine the above. The functionality of the site is a large undertaking and ample time should be afforded to ensure all requirements are identified. There will certainly need to be a good deal of coordination between the organization's IT team and the agency's development team.
Answering these questions will allow you to produce the following: