See the theory description in the Frames to Fields plugin. This effect reads frames at twice the project framerate, combining two input frames into a single interlaced output frame. Effects preceding fields to frames process frames at twice the project frame rate. Each input frame is called a field.
Fields to frames needs to know what field corresponds to what lines in the output frame. The easiest way to figure it out is to try both options in the window. If the input fields are the result of a line doubling process like frames to fields, the wrong setting results in blurrier output. If the input fields are the result of a standards conversion process like 1080 to 480, the wrong setting will not make any difference.
The CINELERRA-GG Community, 2021