What do 16A & 6AX ratings mean on the PDL600RM?

It means the relay module can switch a load of up to 16A resistive or a 6A fluorescent or Inductive  load.

https://www.se.com/nz/en/product/PDL600RM/relay-module-600-series---240-v---6-ax/

Released for:Schneider Electric Australia