What is a question about this article? If the question is unanswerable, say "unanswerable".

If all three parameters are used, they are specified in the following form: [frame size][scanning system][frame or field rate] or [frame size]/[frame or field rate][scanning system].[citation needed] Often, frame size or frame rate can be dropped if its value is implied from context. In this case, the remaining numeric parameter is specified first, followed by the scanning system.

Which two parameters can't be dropped if its value is implied from context?
unanswerable