Here are the main kinds of strict fasts observed in Jainism, with a simple idea of each and noting differences between Digambar and Shwetambar practices:

• Upvas (one-day or short fast): A layperson or Jain practitioner fasts for a day or part of a day. The level of restraint varies, but it typically means eating less than usual, often with no grains and no root vegetables, and sometimes only fruits, milk, or water depending on one’s vow. It is common around holy days and during Paryushan.

• Eka-vrat (single-vow fast): A more formal one-day or few-days fast where a person takes a single vow regarding food intake. Often, the person restricts to specific foods or avoids meals after a certain time (e.g., after noon), and may omit root vegetables and grains. Digambar and Shwetambar traditions describe the vow with slight differences in permitted items and times.

• Bahya-vrat (external restraint): A stricter form of fast where one limits activities and food, sometimes focusing on not eating at all for a portion of the day, while remaining fully celibate and non-harmful to living beings. This is more common among committed lay practitioners during auspicious periods.

• Mahavrat (great vow for monks/nuns): The most stringent fasting observed by Jain monks and nuns. It includes complete restraint from eating (often only water or minimized nourishment), limiting movement, and strictly practicing ahimsa (non-violence). It is a severe, highly disciplined regime not typically adopted by laypeople.

• Sallekhana (voluntary fast unto death): A unique and very strict practice that is undertaken as a conscious, planned, voluntary fasting to purify the soul, usually at the end of life or when one feels no further spiritual progress can be made in the current body. This is a profound and sensitive topic with differing interpretations in Digambar and Shwetambar traditions; it is treated with great care in classical Jain texts.

• Varshik/seasonal fasts (Niyam-vrata during specific periods): Some communities observe longer fasting periods during specific Jain months or holy seasons, with progressively stricter rules about food, water, and activities.

Important notes:

• Digambar vs. Shwetambar differences: Both traditions recognize ekavrat, upvas, and mahavrat, but they can differ in details such as what foods are allowed during a fast, whether water is allowed, and how strictly the vow is observed. If you want, I can outline the exact rules from a Digambar and a Shwetambar standpoint for a particular fast.
• Core aim: The purpose of these fasts is spiritual purification, self-discipline, and renewed focus on non-violence, truth, and detachment.

If you’d like, tell me which fast (e.g., Upvas during Paryushan or Mahavrat for a monk) you’re curious about, and I’ll give you the exact practical rules and the typical timings in that tradition.