Sadhakam has interactive exercises. In the exercises you will hear sequences and will be asked to identify one or more swarasthanams.
In Carnatic music, there are three swarasthanas each for Rishabam (Ri), Ghandaram (Ga), Dhaivatam (Dha), Nishadam (Ni). This is a very fundamental concept and 72 melakartha raga scheme is based on this. Some books wrongly refer to only 2 varieties of Ri, Ga etc., both in text and in notation, either as small Ri, Big Ri or as Ri1 and Ri2 only. This may be right for Hindustani music, but not suitable for Carnatic music.
There's a overlap between Ri and Ga's swarasthanas, and similarly between Dha and Ni swarasthanas. Due to these overlaps, the 16 swarasthanas take 12 distinct positions. Here's a complete list of swarasthanams.
See this blog post for more details.