(literary) this; that; such; used as a demonstrative to refer back to what was just mentioned

Used before a verb to form a noun-like phrase representing the object of the action, meaning "that which" or "what".

extremely; most; to the highest degree (used before adjectives)

(honorific) a polite marker used at the end of formal letters to express a sincere wish or request, typically following a desired action as in 是所至祷 shì​suǒ​zhì​dǎo