Lead Codes – A Detailed Explanation
http://humanesmarts.org/event-report-new-school-of-architecture-fall-fest-at-smarts-farm-2018/three/ The Lead ‘code’ refers to the thickness of the lead. Most lead applications are unsuccessful when the roofing lead used is the incorrect size or thickness. To reduce this risk all rolled lead is manufactured to meet BSEN 12588 with different codes having different applications and weight.
Pirot This table explains how they are categorised.