A domain is the name of a website, and is what comes after the "www." in a web address or after the "@" in an email, for example: testsamplewebsite.com. In your CRM, if the domain is pointed more than once, such as for general use in your funnels, etc. and also for a membership portal, this can create potential issues. It is recommended to have a subdomain for something like a membership portal (in this instance we'd recommend using a subdomain like members.testsamplewebsite.com).
Here's how to set up a subdomain:
Step 1: Under Settings > Domains, connect testsamplewebsite.com with the www. version
Step 2: Under Membership > Settings > Custom Domains you can add the custom domain you want to connect.
Step 3: In the Membership Settings for Custom URL, use the subdomain instead of the main website URL.
Note: The recommended best practice is to have one of the domains in your CRM. Have the other be a re-direct with your DNS manager (such as GoDaddy, Cloudflare, etc.) so that there are no conflicts occurring. For example, have testsamplewebsite.com as the domain in the CRM and within your DNS manager, set up the www. to redirect to testsamplewebsite.com