I'm very curious why i can use local variables in Initial State params and can not do this in Physical section. E.g. why i can't vary density using 700+300*rand($OBJ) expression ?

I believe $OBJ is a string variable that contains the current processed object name and can not be evaluated as a float. Use $OBJID which is a float.

from help:
OBJ – This value is the index of the specific object being processed by the node.

OBJNAME This is a string value containing the name of the object being processed.

I've tried $OBJID, but result is the same