The sort value is an arbitrary number used to determine the order that options appear in. If you leave it as zero, then Kartris will determine the order of options.

So if you want 'colour' to appear after the selection for 'size', set the sort order value to 10 for size and to 20 for colour.

It makes sense to assign in steps of 10 rather than with consecutive numbers, as it gives you the flexibility to insert new values later without having to change all the sort values. For example, you can insert a new option 'style' between size and colour by giving it a sort value of 15.