This page shows all the hooks used in the theme. OsixthreeO uses a standard called the Theme Hook Alliance or THA. THA is old and unattended since 2012 HOWEVER, I think it is still a nice idea.
THA is a good idea because it gives plugin authors a standardized system of where they can put things in a theme. Granted it is not all that useful if only a few themes use it but a theme needs hooks and the THA standard has them in all the needed spots.
The sole argument against THA is that it prevents theme authors from imposing theme “lock-in” on their customers.