what is the difference between share buyback and acquisition of treasury shares? Which one do I need to consider when count share buybacket income ratio?

Contributed Treasury Retained equity shares Reserves earnings $M $M $M $M Total equity $M Notes 4,861.3 (14.2) (4,246.5) 2,123.5 11.1 2,724.1 11.1 6.4 6.4 Balance at 1 January 2020 Statutory profit for the half-year Other comprehensive income recycled on sale of investment Remeasurement of retirement benefit obligations Total comprehensive income for the (4.0) 2.4 (4.0) 13.5 11.1 7 (50.6) (20.6) Dividends paid Reserve arising from IPO Share buy-back Share based payment expense Proceeds from plan participants Issue of shares to plan participants Acquisition of treasury Shares Balance at 30 June 2020 (2.7) 5.8 1.5 6.5 15.6 (13.8) (7.9) (6.5) (4,246.8) (50.6) (2.7) (14.8) 1.5 6.5 1.8 (7.9) 4,840.7 2,084.0 2,671.4