Microsoft Defender - Expert on-demand service

Dear beloved community, 

I read this article https://docs.microsoft.com/en-us/microsoft-365/security/defender-endpoint/configure-microsoft-threat-experts?view=o365-worldwide#consult-a-microsoft-threat-expert-about-suspicious-cybersecurity-activities-in-your-organization and have some questions need to be advised. 

1/ Which license do we have to own for using Microsoft Defender - Expert on-demand service

2/ The Microsoft Defender - Expert on-demand license is for case by case or 1-year subscription

Thanks.