„To post something“ can have many different meanings:

• to post* a letter (to mail** a letter) = to send a letter to someone by post* (mail**)
• to post into a ledger (accounting) = to move debit and credit balances from a sub-ledger into the general ledger
• to post on social media = to write something, for example an article, on social media
• to post something (a poster, for example) = to stick the poster on something, like a message board, using glue, tacks or nails

* = British English
** = American English

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