Onboarding EGS unit

Can anyone Please Explain What this Onboarding Step Actually is
I have read Everywhere a word Onboard Unit Device.

What it actually means Do it means Register an organization on fatoora so that Zatca could check with the CSID that this invoice is coming from a Registered organization.

or do it means something else
And what it is meant by Onboarding more than one EGS unit

what EGS unit Actually is ???

Dear @Shzzamx

we highly advice you to review the educational library shared by ZATCA on the site to have clear understanding, and then share your inquires here.

Thank you