> at the beginning steam seems optional. but I don't know the current
> situation.

Stream is completely optional and distinct. It has nothing to do with the regular CentOS distribution.  It is in addition to regular CentOS and it should not be considered for normal production use.  It’s for developers and vendors.  

https://urldefense.proofpoint.com/v2/url?u=https-3A__lists.centos.org_pipermail_centos-2Dannounce_2019-2DSeptember_023449.html&d=DwIGaQ&c=gRgGjJ3BkIsb5y6s49QqsA&r=gd8BzeSQcySVxr0gDWSEbBYBgsCHS7vSr83lHQ-aa58eXICk1KkzZlAHgohLRRrX&m=H0f2LKhmbbrE_4k0shMDPDyGC6OeEnoOc9R7ZXWGoXA&s=QIzGEmpJ_-Q8yj7G4ZDe4uh3_UZd2b_BF24SN3H3X3w&e= 
https://urldefense.proofpoint.com/v2/url?u=https-3A__developers.redhat.com_blog_2019_09_24_changes-2Dto-2Dcentos-2Dwhat-2Dcentos-2Dstream-2Dmeans-2Dfor-2Ddevelopers_&d=DwIGaQ&c=gRgGjJ3BkIsb5y6s49QqsA&r=gd8BzeSQcySVxr0gDWSEbBYBgsCHS7vSr83lHQ-aa58eXICk1KkzZlAHgohLRRrX&m=H0f2LKhmbbrE_4k0shMDPDyGC6OeEnoOc9R7ZXWGoXA&s=ibEIDID5XuxQsxbbWH3jUMUTkOhx7-rbFncKji35VFg&e=