A subcircuit file has a different syntax and semantics from either the '.asc' file or the '.asy' file. It is also possible to skip the creation of a schematic for lower levels in the hierarchy and just represent that schematic with a subcircuit file.

The schematic has a '.asc' extension, and the symbol has the same name, but with a '.asy' extension. Each lower level in the hierarchy is represented by identically named schematic/symbol pairs. The mechanism for this is that you put together a top level schematic in a '.asc' file. LTspice also introduced the idea of allowing the SPICE simulator to process a hierarchical schematic. It creates a method to draw a symbol on a fixed grid of points and it assigns various 'attributes' to the symbol including 'pin order' numbers and pin names and so forth. thus was created the '.asy' file which is again a text file which does two things. In as much as LTspice has a graphical user interface there was a need to define graphical symbols to represent parts and other circuit elements. You can open, manipulate, and save a '.asc' file with any convenient text editor. It is an ordinary text file without any practical limitation on line length or file size.

This is the file with the '.asc' file extension. Most simulations involve a flat netlist that can be created graphically on a single sheet. No symbols, no graphical user interface, just a deck of 80 column punched cards that you handed in at a window to an operator, and if you were lucky, you got a printout 24 hours later only to find that your deck got booted because of a syntax error. Click to expand.The universe of methods is much richer than you imagine.

Altium designer spice simulation