Arabic Letter Sīn (Isolated Form)

They are the same letter in different encoded forms. ﺱ is fixed in its isolated shape; س is the base letter, whose displayed form changes with neighboring letters. The isolated shape adds no new sound or meaning.

Arabic writing grew from earlier scripts. Nabataean, developed from Imperial Aramaic and generally considered a precursor of Arabic, was used from the second century BCE to the fourth century CE. The fourth century CE Namara inscription is one of the oldest Arabic texts; the 644–645 CE Zuhayr inscription is the earliest dated Arabic inscription.

Syriac is a related Aramaic-derived script tradition, with its own letter ܣ, Semkath. That is a useful historical comparison, not another spelling or encoding of ﺱ.

How do I type the exact isolated form?

Use س for ordinary Arabic; use ﺱ only when the encoded form is required. Arabic text shaping chooses isolated, initial, medial, or final forms from the surrounding letters, so normal input uses the base letter.

This fixed form may not join correctly when placed beside other letters. Different fonts can also draw the same letter with different outlines.