Hi @矢野 尭生 ,
Thanks for reaching out.
Microsoft Entra External ID is next stage evolution, which is designed to seamlessly unite secure and engaging experiences for a wide range of external identities, including customers, partners, citizens within a single, cohesive platform.
whereas B2C is basically to build customer facing applications only.
You can refer this to get more clarity in B2C and External Id for customers here: https://learn.microsoft.com/en-us/entra/external-id/customers/faq-customers
Hope this will help.
Thanks,
Shweta
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