.SUBCKT command in SPICE
.SUBCKT is a macro function, meaning that when encountered it gets substituted.  .LIB is similar but for long term files.

Node names within a .SUBCKT have local scope (Except GND).

Node names on the same line as .SUBCKT are the external node names.

subckt's are called using X prefix.

Your SPICE deck above for top_inv8 is saying, "This is a standalone cell, made up of 3 inv7's and 1 intrpsr_inv8.

Who ever wrote this gave that cell the name of Top, this is the cell that is at the very top level of the design.