What is a Saptak?

In Indian Classical Music, the set of seven notes is called a “Saptak” (derived from the Sanskrit word “Sapt”) that refers to octave in Western music. According to the pitch of sound, Indian Classical Music recognizes three 'Sthanas' or 'Voice Registers' known as 'Mandra Sthana', ‘Madhya-Sthana', and 'Taar-Sthana'. The voice of 'Mandra Sthana' is said to be produced in the chest. The chest voice is the lower, heavier, and more powerful register. The name comes from the sensations you feel in your chest. The voice of 'Madhya-Sthana' is said to be produced by the throat when both the head and chest registers are used simultaneously, it is also referred to as mixed voice.and the voice of 'Taar-Sthana' is said to be produced by the head. The head voice is the higher, lighter, and sweeter register. Sensations are felt in the head. Thus, these three 'Voice-Registers originate from the chest, the throat and the head respectively. 
 
Each Voice-Register is actually an Octave higher than the preceding one. These three Voice-Registers are also known as 'Mandra-Saptak', 'Madhya-Saptak' and 'Taar-Saptak' respectively. 

• Madhya Saptak: the middle octave that the artist choses which is the normal tone for the voice. In the notation system this octave does not have any symbol. 
• Taar Saptak: the octave that is higher than the Maddhya Saptak  and is formed from the notes higher than the ones in Maddhya Saptak (twice the frequency of the swar). The notes from Taar Saptak are represented by a dot above the note. 
• Mandra Saptak: the octave that’s lower than the Maddhya Saptak and is formed from lower notes that are sung in deep low tone. The notes from Mandra Saptak are represented by a dot below the note. 

In Western Music each notation represents an absolute note whereas in Indian Classical Music we use a movable scale (Patti) that can be set anywhere, and all other notes follow a relative route meaning that Sa is an adjustable note which once chosen leads the way for all the other notes. For example: if an artist sets Sa on C then all the other notes are defined in relation to Sa as the consecutive keys in ascending order. This makes the Indian notation easier as it is relative, and the artist can choose any scale as per their tone. 

Picture Ref: https://schoolofindianmusic.com/notation-download/