Indoor porcelain tiles are usually 9–10mm thick. 20mm outdoor porcelain paving is twice as thick and much stronger, so it can be laid on a mortar bed outside and take garden furniture, foot traffic and weather.
You can use some indoor porcelain outdoors if it has a suitable slip rating, but it must be fully bonded with adhesive onto a solid concrete base. It can't be laid on sand or a part-bed like paving. For most patios and paths, 20mm paving is the better choice.