This forum is for reporting errors with the Extraction process. If you want to get tips, or better understand the Extract process, then you should look in the Extract Discussion forum.
Please mark any answers that fixed your problems so others can find the solutions.
I do not quite understand the difference between these two options. Both indicate that they will skip extracting from frames that already exist in the alignments file. If I were to guess:
"Skip Existing": Skips extracting the frame entirely, faces or not.
"Skip Existing Faces": Skips frames, only if there are faces already detected. Otherwise, frame is processed.
Skip Existing - Will not process any frame that exists in alignments file. Would be used for re-generating alignments for a file where "Extract Every N" was used in the past.
Skip Existing Faces - Will attempt to find faces in any frame where faces were not found before.