They identify the sources of the variant readings.
cha = Chaṭṭhasaṅgīti (Burmese Sixth Council)
ma = Maramma (other Burmese)
ka = Kambūjā (Cambodian)
po = Porāṇa (miscellaneous SE Asian palm-leaf MSS)
rā = Rāmañña (Mon)
syā = Syāmaraṭṭha (Thai)
sī = Sīhaḷa (Ceylonese)
yu = Yuropa (PTS romanised)