Wearing grade has nothing to do with whether a tile can be used outside or not. As a general rule, 95% of most porcelain tiles 60x60 or less are frost resistant/frost proof and being covered over is an advantage.
The grade will however dictate how much foot traffic it will take and how much you need to protect it from grit and scratching dirt, and outside that won't be easy. In a domestic garden I would not suggest anything less than grade 4 porcelain but would always push for a 5. And to be honest, I can't remember the last time I saw a porcelain, glazed or otherwise of grade 3 or less..... (scrap that, yes I can.... grade 1..... jet black glazed.... and it still gives me the occasional night terror)
Again, Topps is your first port of call for the grade and you need to ensure they recommend them for external use..... Or buy a new range of tiles that you are confident in.