If "Include in SCIM extension" does not appear it means the attribute is already part of core attributes, ie `urn:ietf:params:scim:schemas:core:2.0:User`.
To learn more visit `https://<your-host>/identity/restv1/scim/v2/Schemas/urn:ietf:params:scim:schemas:core:2.0:User`.
Recommended reading:
- ["How is SCIM data stored"](https://www.gluu.org/docs/gluu-server/user-management/scim2/#how-is-scim-data-stored)
- [RFC 7643](https://tools.ietf.org/html/rfc7643)