It was/is applicable to women of all Varnas except pregnant women and women who have young children.
Practice of Sati was always voluntary in past, although the practice changed a lot due to many historical reasons. Women who escape sati practice by fainting need to observe Prajapatya vow.
Chapter 4, Preta Khanda of Garuda Purana mentions it in detail (translated by MN Dutt)
योषित्पतिव्रता या स्याद्भर्तारं यानुगच्छति ८९
चितिभ्रष्टा तु या मोहात्सा प्राजापत्यमाचरेत् ९०
89(b)-90.A woman who has been chaste and faithful to her husband should mount on the
pyre after bowing to her (deceased) husband before the funeral rites
start. One who gets away from the pyre due to fainting should
observe the vow named präjäpatya.
तिस्रः कोट्योर्धकोटी य यानि लोमानि मानुषे
भर्तारं यानुगच्छति ९१
91.One who ascends the pyre and follows up her husband stays in heaven for a period equal to the number of hair on the body, three
and a half crore.
व्यालग्राही यथा व्यालं बिलादुद्धरते बलात्
तद्वदुद्धृत्य सा नारी
तेनैव सह मोदते ९२
92.Just as the snake-charmer takes out the snake from the hole so also she takes out her husband from hell and enjoys with him in
तत्र सा भर्तृपरमा स्तूयमानाप्सरोगणैः
क्रीडते पतिना सार्धं
यावदिन्द्रा श्चतुर्दश ९३
93.She who ascends the pyre goes to heaven. She is praised by the celestial nymphs and enjoys with her husband so long as the fourteen
Indras rule in heaven successively.
ब्रह्मघ्नो वा कृघ्नो वा मित्त्रिघ्नो वा भवेत्पतिः
नारी तमादाय मृता तु या ९४
94.Even if the man has killed a brähmana or a friend or any other person of noble conduct he is purified of sins by his wife who
ascends his pyre.
मृते भर्तरि या नारी समारोहेद्धुताशनम्
95.A woman who enters fire after the death of her husband prospers in the heaven like Arundhati.
यावच्चाग्नौ मृते पत्यौ स्त्री नात्मानं प्रदाहयेत्
सा हि स्त्रीशरीरात्कथञ्चन ९६
96.Until and unless the woman burns herself after her husband's death she is never released from the bond of her sex.
मातृकं पैतृकं चैव यत्र चैव प्रदीयते
कुलत्रयं पुनात्येषा भर्तारं
97.A woman who follows her husband purifie the three families on her mother's side, the three families on her father's side and the three
families on her husband's side.
आर्तार्ते मुदिते हृष्टा प्रोषिते मलिना कृशा
मृते म्रियेत या पत्यौ
सा स्त्री ज्ञेया पतिव्रता ९८
पृथक् चितां समारुह्य न प्रिया गन्तुमर्हति
सवर्णाश्च आरोहेयुरपीह ताः ९९
चाण्डालीमवधिं कृत्वा ब्राह्मणीतः समो विधिः
अगर्भिणीनां सर्वासामबालताक्मेनामपि १००
98-100. That woman is chaste who
is sad when her husband is sad, who is glad when her husband is glad
and who pines when he is out of station and dies when he is dead.
Common rites are enjoined for all women right from brähmani to Cändäli except for those who are pregnant or who have young