Sharing/outputting to MP-4

Whats the difference between ProRes video and Custom video for sharing please?

ProRes Video is a preset using the ProRes codec. “Custom” lets you choose the codec and its paramaters freely. Basically, the higher level sharing options are to make it easier for users. Use ProRes if you want to use your slideshow for video editing in Final Cut Pro, use “YouTube” if you want to upload to YouTube and so on. The Custom option is there if you need specific settings.