In Jain view, the clear answer is:

Jainism is not a part or branch of “Sanatan Dharma” as commonly used for Hindu/Vedic tradition. But Jain Dharma itself is also called Sanatan – in the sense of being eternal (anādi–ananta).

So there are two different uses of the same word:

1. “Sanatan Dharma” in popular usage (mostly Hindu/Vedic sense)

- Refers to the Vedic/Hindu religious framework. - Jains do not consider Jainism a subset of this. - Jain Dharma has its own: - Tirthankars (like Rishabhdev, Mahavir) - Scriptures (Āgams, Jain Siddhānt) - Philosophy (no creator God, very strict ahimsa, anekāntvāda, aparigraha).

1. “Sanatan” as used inside Jainism

- Jains say: Jain Dharma is Sanatan = eternal, without beginning or end. - It is revealed again and again by the 24 Tirthankars in every time‑cycle. - So Jainism is an independent, eternal dharma, not born out of any other religion.

Summary in simple words:

• Jainism and Hindu “Sanatan Dharma” are two separate paths.
• Both are very old and Indian, some cultural things look similar, but their core beliefs, scriptures and goals are different.
• According to Jain tradition, Jain Dharma itself is Sanatan (eternal), but not a part of Vedic Sanatan Dharma.

You can read a related explanation here