ZBrushCentral

Quick question : what do make polymesh 3d do ?

Hi everybody.
I sculpt a bear, and use decamation master to make a lowres bear ( then i can use the lowres bear to retoply )
I want to export a hi res bear to obj to make a normal map in 3dsmax ( use with the new retopoly low res bear ), but the program say there’s not enough ram ( then i check my free ram : still have 60%+ )
But after i click make polymesh 3d to my bear, then export again, the program work successfully.
So what do make polymesh 3d do ? Why should i click it ? What is the different between a polymesh 3d bear and my previous bear ?