Hi Erik,
The one at the top is for the name, the one in the middle is for the individual, the one at the bottom is for the selected event in the left column.
Usually, general sources are at the individual level if they refer to the whole life of the person.
Otherwise sources are at the even level as most of the time they justify an even (birth, marriage, death, etc).
A source at the name level would only be there to justify the spelling of the name or something like that.
I hope it helps.
Thanks for having read the documentation !
Cheers,
Frederic