clarification

Overall-architecture.md

@@ -179,7 +179,7 @@ server, its IP address and port is configurable in the API.
 
 - **eval** -- evaluate a job. Requires at least 4 frames:
     - `job_id` -- identifier of this job (in ASCII representation -- we avoid endianness issues and also support alphabetic ids)
-    - `header` -- additional header describing worker capabilities. Format must be `header_name=value`, every header shall be in a separate message frame. There is no maximum limit on number of headers. There may be also no headers at all.
+    - `header` -- additional header describing worker capabilities. Format must be `header_name=value`, every header shall be in a separate message frame. There is no maximum limit on number of headers. There may be also no headers at all. A worker is considered suitable for the job if and only if it satisfies all of its headers.
     - empty frame -- frame which contains only empty string and serves only as breakpoint after headers
     - `job_url` -- URI location of archive with job configuration and submitted source code
     - `result_url` -- remote URI where results will be pushed to