Three ways to divide a video
Equal parts suit a recording you want to share in even chunks: three parts of a 12-minute video are four minutes each. A fixed length suits lectures, streams and podcasts with video: every part has the same duration except the last one, which holds whatever remains. Exact times suit a video with natural breaks, such as chapters or songs.
The cards only offer what your video can do. A fixed-length card needs the video to be longer than that length and to come to no more than 50 parts, so a short clip may see just one or two choices.