Required for each additional concurrent* reseller user.
*Concurrent reseller users: A reseller with 10 reseller user licenses (for example) may provide system access to all of their reseller employees (any number of reseller users - sales team, operations, technical, admin, management), however, only up to 10 users can sign on simultaneously. The 11th reseller user signing on will not gain access without an additional user license.
If you require 10 reseller users to access the system simultaneously, you'll need
1 server license + 9 reseller user licenses.
No license is required for end-customer access to the system.