You can sort your photos in contact sheets in multiple ways. Use the dropdown menu to choose your preferred sorting method.

Most sorting methods use the photo's metadata. 

Choose Arrangement to hand sort photos by dragging them wherever you'd like on a contact sheet. Photo Mechanic will remember your arrangement if you toggle to a different sort order and toggle back. 

You can also customize your own sort order by choosing Edit Custom...

Using the Custom Sort dialog, you can edit an existing sort using variables to sort by the metadata properties of your choice.