Providing the 'time lapse'/'single frame' selection helps ACAS upload your files correctly. In the next step you will be asked about your file layout on your disk drive. This facilitates the drag and drop feature.
Later when reports are generated, the report format will change depending on the number of frames found in each well. Any well that contains two or more files is treated as time lapse and the tracked values will be plotted over time. If only a single image file is found in a well, the reports will plot data as single values correlated between the wells.
An overview of the entire file upload process is found here: