[sv-ec] $root vs $unit

What is the diff between $root and $unit ie in below case.
What should be the types of a,b,c.
 
typedef reg T;
module top;
 typedef int T;
 typedef $root.T T1;
 typedef $unit::T T2;
 
 T a;
 T1 b;
 T2 c;
endomdule
 
 
DANiel

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