documentation

Rewritten-docs.md

@@ -197,10 +197,12 @@ users who will be assigned the same set exercises. Each group can have
 multiple supervisors who can manage the students and the list of
 assignments.
 
-Groups can form a tree hierarchy. This is inspired by the hierarchy of
-school classes belonging to the same subject over several school years.
-Supervisors can create subgroups of their groups and further manage
-these subgroups.
+Groups can form a tree hierarchy of arbitrary depth. This is inspired by the 
+hierarchy of school classes belonging to the same subject over several school 
+years. For example, there can be a top level group for a programming class that 
+contains subgroups for every school year. These groups can then by divided into 
+actual student groups with respect to lab attendance. Supervisors can create 
+subgroups of their groups and further manage these subgroups.
 
 ### Exercise
 
@@ -228,8 +230,8 @@ language.
 ### Submission
 
 *A submission* corresponds to a *solution* being evaluated by the
-Backend. A single *solution* can be submitted repeatedly (i.e., when the
-Backend encounters an error).
+Backend. A single *solution* can be submitted repeatedly (e.g., when the
+Backend encounters an error or when the supervisor changes the assignment).
 
 ### Evaluation
 
@@ -249,12 +251,12 @@ runtime environment can be “*Linux + GCC*”, “*Linux + Mono*”, “*Window
 
 ### Limits
 
-A correct *solution* of an *assignment* must obviously master the
-expected behavior and typically must also be effective in some sense.
-The Backend measures the time and memory consumption of the solution
-while running. This consumption of resources can be *limited* and the
-solution will receive fewer points if it exceeds the given limits in
-some test cases defined by the *exercise*.
+A correct *solution* of an *assignment* has to pass all specified tests (mostly 
+checks that it yields the correct output for various inputs) and typically must 
+also be effective in some sense. The Backend measures the time and memory 
+consumption of the solution while running. This consumption of resources can be 
+*limited* and the solution will receive fewer points if it exceeds the given 
+limits in some test cases defined by the *exercise*.
 
 User management
 ---------------
@@ -281,12 +283,49 @@ Exercises database
 
 ### Runtime environments and hardware groups
 
-@todo: describe the concept of RE and HG
+@todo read this later and see if it still makes sense
+
+ReCodEx is designed to utilize a rather diverse set of workers -- there can be 
+differences in many aspects, such as the actual hardware running the worker 
+(which impacts the results of measuring) or installed compilers, interpreters 
+and other tools needed for evaluation. To address these two examples in 
+particular, we assign runtime environments and hardware groups to exercises.
+
+The purpose of runtime environments is to specify which tools (and often also 
+operating system) are required to evaluate a solution of the exercise -- for 
+example, a C# programming exercise can be evaluated on a Linux worker running 
+Mono or a Windows worker with the .NET runtime. Such exercise would be assigned 
+two runtime environments, `Linux+Mono` and `Windows+.NET` (the environment names 
+are arbitrary strings configured by the administrator).
+
+A hardware group is a set of workers that run on similar hardware (e.g. a 
+particular quad-core processor model and a SSD hard drive). Workers are assigned 
+to these groups by the administrator. If this is done correctly, performance 
+measurements of a submission should yield the same results. Thanks to this fact, 
+we can use the same resource limits on every worker in a hardware group. 
+However, limits can differ between runtime environments -- formally speaking, 
+limits are a function of three arguments: an assignment, a hardware group and a 
+runtime environment.
 
 ### Reference solutions
 
-@todo: What it is, what is it used for, how to add one, how to evaluate
-it
+@todo: how to add one, how to evaluate it
+
+The task of determining appropriate resource limits for exercises is difficult 
+to do correctly. To aid exercise authors and group supervisors, ReCodEx supports 
+assigning reference solutions to exercises. Those are example programs that 
+should cover the main approaches to the implementation. For example, searching 
+for an integer in an ordered array can be done with a linear search, or better, 
+using a binary search.
+
+Reference solutions can be evaluated on demand, using a selected hardware group. 
+The evaluation results are stored and can be used later to determine limits. In 
+our example problem, we could configure the limits so that the linear 
+search-based program doesn't finish in time on larger inputs, but a binary 
+search does.
+
+Note that separate reference solutions should be supplied for all supported 
+runtime environments.
 
 ### Exercise assignments