Good news: most FYTA sensors work with mineral substrates and hydro systems too.
Which sensor for which substrate? The Beam G2 and FYTA Mini are compatible with all common growing substrates as well as hydro and semi-hydro systems - including expanded clay, Lechuza-Pon, Seramis, and similar substrates. The FYTA Terra is designed for outdoor soil and isn't suitable for use in pots with mineral substrate.
What soil type should I select? In the app, choose the soil type that most closely matches what you're actually using. The right setting matters because it determines how the app interprets the readings and what watering recommendations it gives you. If you're using a mix (e.g. Seramis + expanded clay), choose the substrate that makes up the largest proportion.