How can I know specific member field in a interface

In seL4 tutorials camkes part, demo program uses seL4DTBHardware connector like this
connection seL4DTBHardware timer_dtb(from timer.dummy_source, to timer.tmr)

next step, in interface init function, the prgram use tmr_0 item as the register space which get from dtb

From the implementation of this program, I know tmr_0 is register space. But how can I konw specific member field in this interface. For example, why _0 in this component is register space

add more picture

add moe picture, about the definition of component