Environments in Power Query. What is k?

Q1.ts

[x = 1, a = [ x = 2,
                  b = [ x = 3,
                        c = [y=1,z=y+x],
                  z = c[z]],
            z = b[z]],
k = a[z]]

Q2.ts

[x = 1, a = [ x = 2,
                  b = [ x = 3,
                        c = [y=1,z=y+@a[x]],
                  z = c[z]],
            z = b[z]],
k = a[z]]

Q3.ts

[x = 1, a = [ x = 2,
                  b = [ x = 3,
                        c = [y=1,z=y+Value.Metadata(@a)[x]],
                  z = c[z]],
            z = b[z]] meta [x=x],
k = a[z]]