What do hub designations in the Obverse and Reverse columns (like notched arrow) mean?
The basic shield nickel design went through several iterations – one major change (removal of the reverse rays) and several minor changes. The hub designations tell you which design hubs were used for the variety in question, and can be an aid to attributions. In the online manual, in the menu section that says References (Links), there is a link to a complete discussion of shield nickel hubs.