The first is common equity Tier 1 (CET1) capital. CET1 includes common shares, stock surplus, retained earnings, other comprehensive income, common shares issued by subsidiaries of the institution, and regulatory adjustments.
The second group is additional Tier 1 (AT1), and it includes instruments that meet the inclusion criteria for AT1 but aren't included in the definition of CET1. These include surpluses from the sale of AT1 instruments, instruments issued by the institution's subsidiaries, and regulatory adjustments.
As you can see, CET1 and AT1 are very similar, and together, they make up the Tier 1 capital. For reference, Tier 2 capital comprises instruments seen as riskier; in the event of financial distress, they are more susceptible to losing their value.
Basel III outlines a framework for calculating RWA based on an assessment of credit risk, market risk, and operational risk. Credit and market risks are calculated using a standard set of approaches applied to instruments identified by Basel III requirements. As such, the accord seeks to apply a framework for the global banking system by defining exactly what goes into the ratio calculation.