$unit vs $root in SystemVerilog

In one line you can say that, $unit represents the top level of each compilation unit, but $root refers to the top level instance.

Now let’s discuss it in little-bit more detail.

$root:

• $root is the root of the instantiation tree.
• SystemVerilog introduced the concept of $root as a global scope that allowed any kind of declaration (data types, classes, variables) along with module definitions nested in that global scope.
• Any un-instantiated module is implicitly instantiated in $root.
• A top-level module is implicitly instantiated once in $root, and its instance name is the same as the module name.

$unit:

• A compilation unit formalizes a scope that represents what is visible in a compilation step – called $unit in SystemVerilog.
• SystemVerilog borrowed the concept of packages from VHDL and standardized the concept of a compilation unit. A package allows you to compile definitions in a separate step and import those definitions into another compilation step.

Basic example of $unit,
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If you have a design that is compiled as a single compilation unit, there is really no conceptual difference between $unit and $root.

However, once you have a design with multiple compilation units, then $unit represents the top level of each compilation unit, and there is nothing in $root except for the implicitly instantiated module instances.

The only time you need to use $root or $unit is when a local name in the current scope hides a name in a higher level scope. For example,

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Here module “mod2” and module “mod1” has same task called “print”. The ambiguity is resolved by giving priority to the local scope and thereby preventing access to the top-level path. $root allows explicit access to the top level in those cases in which the name of the top-level module is insufficient to uniquely identify the path.

Note that there is no way for compilation unit 1 to directly refer to anything in compilation unit 2, or the other way around.

Reference:
1. https://blogs.mentor.com/verificationhorizons/blog/2009/09/25/unit-vs-root/