In the construction industry, Tier 1 is a Contractor referred in ISO 19650 as the Lead appointed party, who provide goods or services directly to the ultimate client.
The Tier 1 are equivalent of the PAS 1192‑2:2013 lead suppliers

In the construction industry, Tier 1 is a Contractor referred in ISO 19650 as the Lead appointed party, who provide goods or services directly to the ultimate client.
The Tier 1 are equivalent of the PAS 1192‑2:2013 lead suppliers
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Electrical & Mechanical Equipment: Access & Maintenance (UK) TL;DR There