In many examples given within the industry of the P-F Curve the spot indicated by a functional failure is almost always close to or next to catastrophic failure. In reality this isn't always the case. The functional failure can only be derived accurately if a proper functional statement (FS) is developed. A functional statement isn't accurate without some performance standards and operating context. Based on the performance standards within the FS a functional failure can be derived.
For instance, if we have a pump that is capable of pumping 600 gpm of water. The process has indicated that they need a desired performance from the pump of at least 500 gpm. Then according to the performance standards within the functional statement the pump is technically failed (Functional Failure) at 499 gpm. The functional failure in this pump is far from catastrophic failure. The pump could actually run for months if not years before catastrophic failure occurs.
It is for this reason, and many others, that the loss of function can only accurately be determined if a proper functional statement is develop for your assets.