Currently _ is not allowed to be part of tenant id.
Apparently RMB and existing modules have allowed the use of underscore. The problem with underscore in this context is that could, under very special circumstances, be mixed with module name.
Say we have module called "users" and another module called "mod-users" (normalized to mod_users).
Tenant diku_mod and module users would use schema diku_mod_users.
Tenant diku and module mod-users would use schema diku_mod_users.