To keep the DMCA safe harbour, 17 U.S.C. 512(i) requires a platform to adopt and reasonably implement a policy of terminating, in appropriate circumstances, the accounts of repeat infringers - and to tell its users about it. A policy on paper that is not applied is how platforms lose the harbour in court. Write the line here; Strikeline applies it the same way to every account.
A strike is a valid takedown against content an account uploaded. A strike is withdrawn only with a reason - the notice turned out invalid, or a counter-notice put the content back. An account that reaches the line is owed a termination; the longer it waits, the weaker "reasonably implemented" looks. A terminated account takes no more strikes and is not reinstated.