The "Initial Run Options" dialog that appears next is there to try to save you time by avoiding recopying files that may already exist on both Folder A and Folder B but have different date/times.
Consider files whose name and size match to be identical - You may want to use this option if there are existing files on both Folder A and Folder B that you know are the same but whose last modified date may differ by months or years. (This is the default option.)
Consider files whose name and size match to be identical only if the timestamps on A and B are within X (units) of each other - This option resembles the previous "Consider..." option except it gives to the ability to say that files older than a particular age need to be copied even if they have the same name and size. (A setting of "0 minutes" really make this option behave like "Simply copy...")
Simply copy all files from A to B - This replaces all files on Folder B with fresh copies of files from Folder A. As the label suggests, this option takes the longest of any of the three options.
If Folder A or Folder B is empty, your selection will have no effect. Files with the same name on Folder A and Folder B but having different sizes will always be overwritten, regardless of your selection. (Your selection only affects existing files with the same name and the same size.)
