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Any action which we perform according to the prescription of scriptures. That is also once Sva dharma or to be very specific Varnasharma Dharma according to its Varna & Ashram which is not based on birth but based on Guna or Svabhav ( Reference : BG 4.13
Actions not prescribed in scriptures. All wrongful activities which breaks the principles of Mercy , Austerity , Cleanliness and Truthfulness comes under this category . Reference : BG 4.17
Any action which does not create any reaction. But how it is even possible ? All activities performed in pursuance to Yoga in order to attain Moksha comes under this ( BG 4.18) . This is very difficult and interesting category to understand . Akarma even destroys the existing reactions of past Karma. Proof : there are 2 verses in Bhagavad Gita which explains this phenomenon: BG 2.40 and 18.66