Instances and licenses

Rewritten-docs.md

@@ -504,7 +504,22 @@ required to get a credit. These points can be get by solving assigned exercises.
 To visualy show users if they already have enough points, ReCodEx groups
 supports setting this limit. There are two equal ways how to set a limit --
 absolute value or relative value to maximum. The latter way seems nicer, so it
-is implemented. The relative value is set in percents and is called threashold.
+is implemented. The relative value is set in percents and is called threshold.
+
+Our university has a few partner grammar schools. There were an idea, that they
+could use CodEx for teaching informatics classes. To make the setup simple for
+them, all the software and hardware would be provided by university and hosted
+in their datacentre. However, CodEx were not prepared to support this kind of
+usage and no one had time to manage a separate instance. With ReCodEx it is
+possible to offer hosted environment as a service to other subjects. The concept
+we figured out is based on user and group separation inside the system. There
+are multiple _instances_ in the system, which means unit of separation. Each
+instance has own set of users and groups, exercises can be optionally shared.
+Evaluation backend is common for all instances. To keep track of active
+instances and paying customers, each instance must have a valid _license_ to
+allow users submit their solutions. License is granted for defined period of
+time and can be revoked in advance if the subject do not keep approved terms and
+conditions.
 
 @todo: explain instances why they are useful what they solve and also discuss licenses concept