ص different from tsade
Significance
In gematria, ṣadi represents the number 90. Its final form represents 900, but this is rarely used, taw, taw, and qof (400+400+100) being used instead.
As an abbreviation, it stands for ṣafon, north.
Ṣadi is also one of the seven letters that receive a special crown (called tagin) when written in a Sefer Torah. See shin, ‘ayin, tet, nun, zayin, and gimmel.