Difference between instance and reference

Units are like a static struct, they can only be generated during pre-run generation and they have
a static path in the hierarchy (e-path) which doesn't change during a run (units have other specific attributes
like hdl_path()). The recommendation is to use units to model static test bench components (monitors,
BFMs, sequence drivers etc.) which are valid throughout the test.

structs can be dynamically generated (on the fly), they don't have a static e-path etc.

unit abc {}; struct xyz {}:
abc_r : abc; (Reference) -> reference(pointer) to a static unit
abc_i : abc is instance; (instance) -> static unit instance
xyz_r : xyz; (Reference) -> this is a struct instance, not a reference
xyz_i : xyz is instance; (instance) -> this is an error, can not be done

to get a reference to xyz_r you could do:
var xyz_ref: xyz;
xyz_ref = xyz_r;
(xyz_ref was not generated and just assigned, so it is a reference to xyz_r)

In general assignments of structs to other structs are done by reference. After the assignment
takes place, the variable on the left side refers to the same instance that was referred to by the
expression on the right side.

-hannes