What is the difference between Aperture photo index number and Aperture photo sequence number?

I'd like to understand what photo "sequence number" is and what photo "index number" is.

• A "sequence number" format will create names with numbers within a number range starting by one, for example "myname (10 of 20)", if you are importing 20 images.
• An "index number" format will create a single number, without the "of 20", and the last index can be kept as starting point for the next import, if you want to start with the last number.

You can see, what the predefined formats are doing, if you set the "Rename Files" to "Edit". The edit panel will show you an example filename and you can configure the format string to your own needs, by dragging items from the palette to the string. Use the "+" button to create your own preset, if you do not want to modify the pre fined presets.