All subcircuit and model names are considered global and must be unique. The .param symbols that are defined outside of any .subckt ... .ends section are global. Inside such a section, the pertaining params: symbols and any .param assignments are considered local: they mask any global identical names, until the .ends line is encountered. You cannot reassign to a global number inside a .subckt, a local copy is created instead. Scope nesting works up to a level of 10. For example, if the main circuit calls A that has a formal parameter xx, A calls B that has a param. xx, and B calls C that also has a formal param. xx, there will be three versions of `xx' in the symbol table but only the most local one - belonging to C - is visible.