ARROW INDICATES WHAT??????????

HI
in HFSS,PROJECT MANGER window,under PLANES,if i press any one plane (example:Global:XY) aplane is comming in XY plane and one ARROW comming in the +ve Z-direction.what indicates this arrow,what is the meaninig of this ARROW.
please help me ASAP........
THANKS

It is a vector perpendicular ("normal") to the plane. In vector math, planes are normally described in terms of this normal vector.
http://en.wikipedia.org/wiki/Surface_normal

Not sure why HFSS shows it, perhaps some people find it useful.