Many assertions are over concrete state. For instance `Never is light A off when light B is on' .
Other assertions need to refer to symbolic values. For instance `The value in register X is always less than the value in register Y' .
State properties describe the current state only. For instance `Light A is off and light B is on'.
Path properties relate successive state properties to each other. For instance `light A always goes off before light B comes on '. We shall see PSL requires the symbolic values be embedded in the bottommost `modelling layer' and that its temporal layer cannot deal with symbolic values. For instance, we cannot write `{ A(x);B(y) } |=> { C(x,y) }'.
(Note: the internal representation used by a checker tool for a concrete property can commonly use a symbolic encoding, such as a BDD, to handle an exponentially-large state space using reasonable memory, but that is another matter.)