This would apply, for example, if you need to copy a copyrighted document for security or preservation purposes. Sometimes an architectural plan, if it fits into the pictorial works category, may also fall within one of the exceptions to the exceptions rules and you cannot legally make a copy. You really have to know the date of the document and understand which category it falls under. An institution can make copies for specific users on a one-time basis, with certain limitations. Circular 21, provided by the Copyright Office, covers these details.
Fair use comes into play when there is an actual claim of infringement.
This doctrine developed from common law, so although it is codified, it tends to be very loose. You may make a fair use claim if a work is used in connection with criticism or comment on the work, in the course of news reporting, for teaching purposes, or as part of scholarship or research activity. If you are using someone else's material from a web site, it will be considered infringement. A contributory or vicarious infringement occurs when you are inducing or in some way profiting from a link. These new technologies are giving rise to new laws.