Panel a: diagram of APOGEE [Al/Fe] vs. [Fe/H] for RGB stars in ωCen. Panel b1: zoom-in on the iron-poor end. Stars with ChM tagging are colored according to their assigned population, while the vertical dot-dashed line marks the 90th percentile of the 1P-star [Fe/H] distribution. Panel b2: normalized kernel-density distribution of [Al/Fe] for all stars (black) and for 2P stars (violet). The two dot-dashed horizontal green lines indicate the [Al/Fe] range of 1P stars. Panel c: same as panel b1 but zoomed on higher [Fe/H]. Panel d: [Al/Fe] vs. [Fe/H] For APOGEE stars located beyond 5 arcmin, colored according to the population tagging described in Section 3.2. The black lines mark the boundaries between the three streams, as reported in the plot.