SelectFree[expr, a, b, ...] is equivalent to Select[expr, FreeQ2[#, {a,b, ...}]&], except the special cases: SelectFree[a, b] returns a and SelectFree[a,a] returns 1 (where a is not a product or a sum).
Overview, FreeQ2, SelectNotFree.
SelectFree[a + b + f[a] + d, a]b+d
SelectFree[x y, x]y
SelectFree[2 x y z f[x], {x, y}]2 z
SelectFree[a, b]a
SelectFree[a, a]1
SelectFree[1, c]1
SelectFree[f[x], x]1