Why can I only add 1 user in the standard plan if you add multiple domains?

Why can I only add 1 user in the standard plan if you add multiple domains?

Hi. I really need some help.
Excited after signing the free plan, I decided to sign the Standard plan
Because I understood that I could add some domains and create the corresponding email boxes until I get to 25 and having a total of 30gb

FREE: 25 Users + up to 25 referral bonus users
5GB** Storage is counted as combined storage for Zoho Mail and Zoho Docs.

STANDARD: $ 2 /User /Month
Everything in FREE+
Multiple domain hosting.
Domain aliases.
Additional storage upgrade.

Why can I only add 1 user in the standard plan if you add multiple domains?

I'm trying to understand here,
In the above description said it could have "" and more 30gb?

Is not the email box the same as the user?
Please explain this to me!
Why would I have 3 domains if I can only have one email box?