task types in job config

Rewritten-docs.md

@@ -1010,6 +1010,29 @@ existing parsers for most of the programming languages and it is easy enough to
 learn and understand. Another choice which make sense is JSON but at the end
 YAML seemed to be better.
 
+#### Task Types
+
+From the low-level point of view there are only two types of tasks in the job.
+First ones are doing some internal operation which should work on all platforms
+or operating systems same way. Second type of tasks are external ones which are
+executing external binary.
+
+Internal tasks should handle at least these operations:
+
+- *fetch* -- fetch single file from fileserver
+- *copy* -- copy file between directories
+- *remove* -- remove single file or folder
+- *extract* -- extract files from downloaded archive
+
+These internal operations are essential but many more can be eventually
+implemented. Full list should be accessible in 'Job configuration' appendix.
+
+External tasks executing external binary should be optionally runnable in
+sandbox. But for security sake there is no reason to execute them outside of
+sandbox. So all external tasks are executed within sandbox which should be
+general and configurable. Configuration options for sandboxes will be called
+limits and there can be specified for example time or memory limits.
+
 #### Configuration File Content
 
 @todo: discuss what should be in configuration: limits, dependencies, priorities... whatever