Compare the Salesforce Identity, External Identity, Customer Community and Partner Community licences.

• Salesforce Identity is for employees whose relationship with Salesforce is identity rather than CRM — single sign-on out to third-party applications, an App Launcher tile, connected app provisioning — without full CRM object access.
• External Identity is for consumer-scale external identity: registration, login, profile and consent management, with deliberately limited data access.
• Customer Community is the high-volume external licence. Those users have no role, which is why they cannot appear in sharing rules and why sharing sets and share groups exist for them.
• Customer Community Plus and Partner Community are role-based: users sit in an external role hierarchy under their account, participate in sharing rules, and get broader functionality. Partner Community adds the sales-oriented objects partners need.

The architectural point is that the licence sets a ceiling a permission set cannot lift. So the licence question belongs in requirements, not in build. “Can these External Identity users also work cases?” asked at the end of a project means re-licensing a live population; asked at the start it is a five-minute conversation. And because entitlements are revised over time, I confirm the current terms in the licence documentation rather than quoting figures from memory.