Runtime Server as the name implies is used to execute models. Its main purpose is to run models, it can not be used for creating models and modify models.
Runtime Servers license is purchased by server instance. This is different from ProMBT and Enterprise editions which are licensed to a named user.
A server instance is defined as one instance of TestOptimal server running on the licensed host on the specific port#. Each server instance running on the same host but on different port# is considered a separate license instance.
Another license type for Runtime Server is float license. This is typically used in VM environment where Runtime Server is installed and one or multiple instances may be started. Float license is purchase by server instance and the license is not locked down to a specific host or port#. Float license requires the purchase of Server Manager which offers additional features in server management and model deployment.