what is the difference between C, cc, and S commands in vim

Question

They seems to erase the line and enter the insertion mode.

Answer 1

• C replaces the rest of the line (starting at cursor position) with your edit.
• cc and S are synonyms and replace the whole line(s) with your edit.

Answer 2

From :h C...

Delete from the cursor position to the end of the line [...] and start insert. Synonym for c$.

And :h S gives us...

Delete [count] lines [...] and start insert. Synonym for cc.