The biography uses mostly 'secondary' sources. [=information that did not come from original/primary sources but came from research done by others]
There are some 'secondary' issues/considerations which must be taken into account as well.
In some children's books, the text is 'secondary to' the illustrations.
(..) but... I pulled my gun once. Shot it once. - Really? - It was my first one of these. We were a secondary unit. I was pretty shaky going in. I was a rookie. We bust open the door looking for this junkie, the fucker opened fire on us. One cop got hit in the arm. (..)1995 Se7en