Under the option: Choose which external domains your users have access to:
Allow all external domains: This is the default setting in Teams, and it lets people in your organization find, call, chat, and set up meetings with people external to your organization in any domain.
Allow only specific external domains: By adding domains to an Allow list, you limit external access to only the allowed domains. Once you set up a list of allowed domains, all other domains will be blocked. To allow specific domains, click Add a domain, add the domain name, click Action to take on this domain, and then select Allowed.
Block specific domains - By adding domains to a Block list, you can communicate with all external domains except the ones you've blocked. To block specific domains, click Add a domain, add the domain name, click Action to take on this domain, and then select Blocked. Once you set up a list of blocked domains, all other domains will be allowed.
Block all external domains - Prevents people in your organization from finding, calling, chatting, and setting up meetings with people external to your organization in any domain.
Choose the option you wish to select carefully - the company may have a policy in place that restricts access to other Teams/Skype for Business users, or they may restrict it to a certain domain. Altering this setting will have a global effect on every user - make sure this goes through change control process