Ch 18, Sh 42-45, Bhagawad Gita, Shankar Bhashya

Svabhavajam brahma-karma, the natural duties of the Brhamanas, of the Brahmana caste; are samah, control of the internal organs; damah, control of the external organs-these bear the meanings as explained earlier (see 6.3, 10.4, 16.1); tapah, austerity-bodily austerity, as explained before (17.14); saucam, purity, as already explained (in 13.7, 16.3); ksantih, forgiveness; arjavam, straightforwardness, simplicity; jnanam, knowledge; eva ca, as also vijnanam, abiding in the knowledge; astikyam, faith, the idea of truth [Truth of the scriptures, existence of God, etc. asti-bhavah astika-bhavah, the feeling of conviction with regard to the existence of God and the other world, respect for the teaching of the scriptures. By svabhavajam (natural) is conveyed the very same idea as was expressed in 'classified according to the gunas born from Nature' (41).
Svabhavajam, the natural; ksatra-karma, [A variant reading is ksatram karma.] enjoined duties of the Ksatriyas, of the Ksatriya caste; are sauryam, valor; tejah, boldness; dhrtih, fortitude, as is seen in the case of one who is not depressed under all circumstances, being sustained by doggedness; daksyam, capability of engagement without confusion in duties which suddenly present themselves; api ca, and also; apalayanam, not retreating; yuddhe, from battle, not fleeing from enemies; danam, generosity, distribution with free hands; isvarabhavah, lordliness, manifesting (exercising) rulership over those who have to be ruled.
Svabyavajam, the natural; vaisya-karma, duties of the Vaisyas, of the Vaisya caste; are krsi-gauraksyavanijyam, agriculture, cattle rearing and trade: Krsi is tilling of land. Or who rears cattle (go) is goraksa; the abstract form of that word is gauraksyam, animal-husbandry. Vanijyam means the occupation of a trader, consisting of buying and selling. Sudrasya, of the Sudra; api, too; svabhavajam, the natural; karma, duty; is paricaryatmakam, in the form of service.
When rightly pursued, the natural result of these duties enjoined for the castes is the attainment of heaven which act is evident from such Smrti texts as, 'People belonging to the castes and stages of life, who are true to their own duties, experience after death the fruit of their actions. And after that, as a result of the remnants of their merits they are born in some excellent region, caste and family, with greater piety, longevity, learning, conduct, wealth, happiness and intelligence' (Ap. Dh. Su. 2.2.2.3), etc. And in the Puranas also it is particularly mentioned that people belonging to the (different) castes and stages of life come to have specific results in the form of different worlds.
But this result that is going to be stated follows from a different cause:
Sve sve karmani abhiratah, being devoted to his own duty, which has different characteristics as stated above; narah, man, the person qualified therefor; labhate, attains; samsiddhim, complete success, characterized as the ability for steadfastness in Knowledge, which follows from the elimination of the impurities of body and mind as a result of fulfilling his own duty.
Does the complete success follow merely from the fulfilment of one's own duty?
No. How then?
Srnu, hear; tat, that; yatha, as to how, through what means; sva-karma-niratah, one devoted to his own duty; vindati, acheives; siddim, success.