Each remote device, connected to WebHMI always has some settings or setup data which does not have to be read in each WebHMI scan. They are actually accessed by operator only during commisioning or installation.

E.g. temperature requlator may have only 2-3 process related data ( current temperature, setpoint, control output value), but tens of even hundreds of setting registers, like sensor types, calibation data, control profile ( ON-OFF, PWN, analog control, PID coeficients, hysteresys) etc.

So when you add many such controlles with all registers to the WebHMI project, it will communicate 90% of time in vain, and only 10% to read important data.

To avoid this situation, there a special attribute for the registers which marks them as “Read on-demand” so that WebHMI skips them when polling external devices.