You can not be a subreseller. The option to have clients is not available then.
Yes - I've got that point a few days ago. It is confusing - Other systems (including ISPConfig2) seperate this more clearly.
We cana make a copy of the menu point named reseller with a list that shows only clients that have clients if everyone is confused by this.
Personally I dont like it to dupliacte the same information in two places.