12c Out of the Box Metrics

 I am new to OEM and was wondering how these template metrics work.  For example there is one called Archiver Hung Alert Log Error.  When editing there  is a filter expression which I assumes is telling OEM to ignore whats in the filter.  Then there is Monitored Objects which has one line "All Others" can Critical Threshold set to ORA-.  my questions are as follows:

1.  What is timeline/Number indicate?

2.  Does this metric automatically look for the ORA- errors which indicate Archiver Hung?

3.  Why are the same ORA- errors in every filter (ORA-54,ORA-1142,ORA-1146?

4.  Is there any documentation that explains this?