Tier and hosting
For a hosting customer, this means a stronger backend for continuity of websites, e-mail, DNS and backups.
It does not replace good application practices, but it provides a stronger infrastructure foundation.
Careful wording
Public DataHouse materials mention Tier 3 ANSI/TIA-942 requirements and the newest part meeting Tier 4 requirements.
On Hostilla.pl we describe the meaning of these standards without promising parameters that should come from a specific SLA agreement.