This article will help you understand the sign-in process for CUBE.
The portal referred to here is the ‘Company Portal’. Within CUBE, a company can utilize multiple portals. You might be a part of either a single portal or many at the same time with the same email ID.
If you have signed up for CUBE and have been added to a portal, you can log in to that portal. Here’s how:

A Subdomain Link is a direct login link specific to a single portal or workspace.
Each portal has its own unique subdomain (for example: acme.app.cube.ms).
When a user opens a subdomain link, they are taken directly to that specific portal’s login page.
Upon successful authentication, the user is logged directly into that portal, without needing to choose from other portals they may belong to.
If you are on subdomain.app.cube.ms and enter a wrong email address, then this error will be shown. - This email is not associated with any user on this portal. Please check the email and the portal URL and try again.

On the next page:

Upon entering the correct password and clicking on SIGN IN, you will log into the portal dashboard directly, even if you are part of multiple portals, as you were on a subdomain link.

The App Link is a universal login link for all users.
It does not belong to a specific portal; instead, it serves as a common entry point to CUBE.