Definition of tiers:
(n) :
A row or range, especially one at a higher or lower level than another.
(n) :
A rank or grade; a stratum.
(v) :
(transitive) To arrange in layers.
(v) :
(transitive) To cascade in an overlapping sequence.
(v) :
(transitive, computing) To move (data) from one storage medium to another as an optimization, based on how frequently it is accessed.
(n) :
Something that ties.