The Remove Before Flight tag is Kennedy Space Center form 4-226(6/77).
This 3-inch wide by 1-foot tall tag and ones like it are used to flag support hardware, protective covers and other items temporarily attached to the orbiter and/or its payloads that are not intended for flight. The red color makes it easier for engineers to see and insure that all equipment that should be removed is done so before the shuttle launches for orbit.
