Content of the Entities Section

Looking for some clarification on the content of the $Entities section of the .msh file. I'd be grateful for any response to the following:

1. The first portion of the section (from the online manual): numPoints(size_t) numCurves(size_t) numSurfaces(size_t) numVolumes(size_t) pointTag(int) X(double) Y(double) Z(double) numPhysicalTags(size_t) physicalTag(int) ...

What exactly are these entries describing?

2. In the remaining portions of the section, two coordinate triplets are listed. I'm not sure what this data represents; are these the coordinates of the associated bounding box for the entity?
3. In looking at a sample .msh file, some of the lower-order entity tags are positive while some are negative; what does this represent?

Thanking you in advance,