hi there, instance and occurrence metrics are used with eVars. What they means is that how many times that eVar was set in an image request. For e.g. when a customer visits through a paid search and it is captured in an eVarX, the eVarx is fired at that point and instance and visit are recorded as 1.
if the person visits again coming directly (Typed or Bookmark) and eVarX is NOT fired, now the instance is 1 and visits are 2.
Instances are based on how many times an eVar was set by your implementation.
Hope it explains.