Thickness
Tiles sold for indoor flooring are generally 10mm thick. This is ample for the purpose, as they are laid on a solid base using tile adhesive.
Outdoor porcelain tiles are manufactured to 20mm thickness, while natural stone can be as thick as 25mm, or more in some cases.
Laying exterior-thickness tiles and flags indoors runs the risk of creating problems with door clearance and skirting boards. In addition, many people lay tiled floors over underfloor heating. Thicker porcelain and stone will take longer to heat up and, though you may find that it also radiates heat for longer when the heating goes off, more energy is required before you can enjoy the toe-warming benefits.
Taking indoor tiles outdoors can also present problems. At 10mm thick, even with a really good foundation they are unlikely to withstand heavy garden machinery or furniture. They’ll also be exposed to rain, frost, ice or snow. As they’ll be less frost-resistant, you are likely to have the disappointment of seeing tiles crack, split and delaminate.