Radius ratio

Don't you think that depends one heckuva lot on exactly what crystal you are talking about and what the atoms are?

Unless you are asking about the difference in meaning and I would think that would be obvious!

 

The bulk of a crystal is a repeating pattern,
so a "unit cell" can be centered on any atom.
There are many different types of "unit cells",
some retained for the sake of tradition.

Whenever the book describes structure vectors,
assemble a set of sticks in 3-d to show them.
Especially at first, 2-d drawings might not cut it.